In 1780 the 
Hofund Kammergericht  (like the Justice Department) of the Prussian government requested  that the Berlin Jewish community under the jurisdiction of its Chief  Rabbi Hirschel Lewin 
[1]  furnish for them a digest of the Jewish marriage, financial and  inheritance laws. The purpose of this tract was to supply the state with  a working knowledge of Jewish laws. At the time there was ferment in  the direction of incorporating Jews into the state, and following the  direction of the Jewish rights' advocate 
Dohm  there was some interest in learning how to apply Jewish law to judge  cases brought to secular courts on appeal (that is, after failure of a  Bet Din to reach a satisfactory conclusion) according to Jewish law. As  wild as this scheme sounds, it was probably sincere, although  adjudicating cases between Jews according to  Jewish law could not have been its only purpose. Another reason was  probably to learn if Jewish  laws were compatible with secular law. (Similar works were requested at  other times. For example, the author of the responsa Noda Beyehuda,  Rabbi Ezekiel Landau, produced a work on Jewish marriage law called 
Das  mosaisch-talmudisch Eherecht for the Emperor Joseph II.)
Rabbi  Lewin was requested to produce it in two weeks, but it didn't appear  until six years later. It was then published in 1778 under the title 
Ritualgesetze der Juden. Judging by  the title page this work seemed to be a collaboration between Lewin and Moses  Mendelssohn, in fact it was really the work of Mendelssohn. The rabbi,  who was a good friend of Mendelssohn, only reviewed it and made  necessary corrections.
Below are two reviews of this work  (admittedly they don't tell us much) which appeared in British  periodicals:
1.

2.

Rabbi Lewin was a good friend of Mendelssohn's. He was among the only  rabbis whom Mendelssohn asked for an approbation for his Pentateuch  translation, and certainly among the only rabbis  at the time to specifically concur with the need for a a modern Jewish  translation into the vernacular. The complete text of his 
haskamah to Mendelssohn's Biur (ie  
Chumash Nesivaus Scholaum) was  only included in the very first edition of 1783 (at the beginning of the  2nd volume, Exodus).  Excerpts from it were published in Landshuth's biography of Lewin. The  entire piece, however, was printed at the beginning of the Romm edition  printed in Vilna 1849. Here is the original followed by the more  readable Romm version:




This approbation includes the observation that the Gentiles believe in  the Torah and prophets, understood them and have made vernacular  translations of the Bible. Yet the Jews, especially the Ashkenazim,  don't speak a clear language (*coughYiddishcough*). He writes that he's  seen a Yiddish Bible printed in Amsterdam in 1679, and also another one,  both of which are greatly wanting, but they were the only ones available  for Jews who wanted to understand Scripture. However, there were the  impressive non-Jewish translations, and these are stumbling-blocks for  young people. So it is that an honorable rabbi, scholar and sage, famous  and expert in Torah, Talmud, science, Hebrew grammar and the German  language, not one in a thousand is like him -- Mendelssohn -- has  written such a needed translation . . .
Although this approbation was written in Berlin in 1778, it has been  noted that their friendship dates at least as far back as 1764 - 1770,  when Lewin served as rabbi in Halberstadt. In Geiger's JZWL 1872 pg. 232  a letter is printed, written in August, 1770, in which the poet Gleim  informs F. E. Boyzen that Rabbi Lewin ("Herr Loebel") was an admirer and  friend of Mendelssohn. (This letter is also reproduced in Latin letters  in Landshuth's Toldos Anshe Hashem U-peulosam be-adass Berlin 
pg.  83) There is a possible earlier connection: in 1756 a German  translation of sermon of Rabbi David Frankel, author of the Korban  ha-Edah commentary on the Jerusalem Talmud, was printed. Shortly  thereafter followed an English translation, which was reprinted many  times, including in America! Rabbi Frankel was Mendelssohn's rebbe; he  was also Rabbi Lewin's first cousin. It was, in fact, Mendelssohn  himself who had made the German translation of Rabbi Frankel's sermon  (see Gad Freudenthal's article in the EJJS 1 on this sermon). At the  time Rabbi Hirsch Lewin was beginning to serve as Rabbi Hart Lyon in  London, where the English version of the sermon appeared. Although I've  no proof to give of any involvement on the part of the rabbi, much less  that he and Mendelssohn knew one another yet, one might say that their  circles overlapped even at this early period, although Lewin was eight  years older than Mendelssohn.
Here's a notice of the rabbi's leave of England toward his post in  Halberstadt (Gazetteer and New Daily Advertiser (London  England) Wednesday May 23 1764):

Ultimately their  friendship was tested over a notorious incident, namely the Wessely 
Divre Shalom ve-Emes affair.  Demonstrating that one can't always predict affinities and affiliations  -- as I will elaborate -- Rabbi Lewin was outraged by Wessely's pamphlet  in which he called for a restructuring of the Jewish educational system  (or some might say, he called for 
structuring  that 
"system") and insulted  the rabbis. He was so upset that he sought to toss Wessely out of  Berlin. Wessely was Mendelssohn's close friend, and their varying  viewpoints came between the rabbi and the philosopher. Eventually Lewin  made it clear that either  Wessely goes or he goes. In fact, neither went.
What I meant by  not being able to predict affinities was that one of 
Mendelssohn's  fiercest rabbinic critics, Rabbi Elazar Fleckeles, the foremost student  of the Noda Beyehuda, appears to have held a fairly mild, and even  positive  view of Wessely. In addition to quoting him several times in his  writings, it seems they also had a correspondence, although I'm not sure  how extensive. In 1800 Rabbi Fleckeles published 
Ahavat David, a stridently  anti-Frankist/ anti-Sabbatian tract, based upon sermons he had given on  the subject (see this excellent 
post  by Rabbi Eliezer Brodt). The book included a 1796 letter from Wessely,  which Fleckeles offered as evidence that Rabbi Jonathan Eybeschutz was  innocent of Sabbatianism! I'll get to the content of the letter in a  moment, but first I will just note that I examined the pdf of Ahavas  David twice, and I was unable to find the letter. I couldn't understand  why I couldn't find it, so I asked several friends (literary men) if  they knew what page it was on. Dr. Marc Shapiro replied that this  edition -- a Copy Corner reprint -- is censored; it doesn't include the  letter! I suppose it's theoretically possible that the censorship, ie,  removal of the letter occurred long before the scanning of the book.  That is to say, there's no way at the moment to tell who removed it and  when, but we can probably guess why. In any case, at the same time I  asked Shapiro for clarification, Rabbi Brodt told me that 
his copy of  Ahavas David includes the letter, and he sent it to me. Here it is:


As you can see, Wessely writes:   
"By  my life, I heard in my youth from the mouth of the great kabbalist, who  knew the Zohar and all the works of the Ari by heart, the rabbi, my  master, Rabbi Jonathan Eybeshutz ZZ"L, that he used to say to his  audience when they were hesitant to accept a kabbalistic teaching, 'if  you don't believe it, it's no matter, because it isn't from the  fundamentals of faith.' So he used to say to those who brought  kabbalistic teachings to explain a piece of Gemara or Midrash, 'I don't  desire this. What's the use? According to kabbalah you can explain  anything you want to; just tell me the simple meaning via "niglah"' -- it's completely true!"
   One imagines that Rabbi Fleckeles sought such information which had the  two-fold advantage of showing that a great Kabbalist like Rabbi  Eybeschutz ultimately marginalized kabbalah, something which was  important in combating Frankists and the vestigial Sabbatean movement.  Secondly, it also strongly suggested that Rabbi Eybeschutz himself was  innocent of the charge that he himself was a secret Sabbatean. As a  student of Eybeschutz, Wessely was in position to recall his master's  views for the benefit of Rabbi Fleckeles, nearly 30 years his junior,  who was only 10 years old when Rabbi Jonathan Eybeschutz died.
Actually,  it's a crude case of censorship, because the eagle-eyed reader could  notice that the last page of the introduction promises that the first  word on the following page will be 
האלדים. Instead the  first word is 
אשר. As you can see, when these two pages are  inserted properly the leading words are quite correct:




In  any case, the complete letter was also printed in 
Hameliz 48 pg. 750, May 7, 1886:

Finally, it doesn't seem inappropriate to add something interesting  relating to the controversial son of Rabbi Lewin, Saul Berlin  (1740-1794), known as the forger of the Responsa Besamim Rosh. In 1844  an old man living in London named Meyer Joseph submitted a piece for  Julius Furst's Literaturblatt des Orients. Joseph had been a close  friend of Saul Berlin in his youth:
It was in the year 1794 when this exceptional man died here, and I think  I have the right to publish this article as I was the only friend he  had here. He was on a long journey, the object of which I do not  remember anymore, and intended also to stay in London for some time. I  visited him daily, we remained often together for hours at a time, and,  although I am now 83 years old, the impression he made upon me, his  eloquence and his whole personality remain unforgettable to me. A few  months after his arrival he fell ill with cramp and it was I who closed  his dying eyes. On his death the London community paid him respect. He  was buried with great honors on the 25th of Marcheshvan, 1794. On  arranging the things he left behind him I found his will, which I then  copied for myself. (translation by David S. Katz in 'The Jews in the  History of England,' 1485-1850, pg. 325, except that he translated  "Heshvan" while I preserved the "Marcheshwan" of Joseph's original) The  piece included the will, which Joseph himself copied:

Writing to the Jewish Chronicle on October 4, 1935, Cecil Roth noted  that as Yom Kippur approaches, it should be noted that in "most London  synagogues" there is Yizkar recitation of a list of rabbis, but it  seemed this custom was on the wane. So as to "refresh [the] memories"  Roth included a list of the 10 rabbis. Number 6 is  Rabbi Saul ben Rabbi  Zevi, ie, Saul Berlin. His name is included in the list "out of  compliment to his father, and in view of the fact that he died in  London."
[1] Rabbi Tzvi Hirsch ben  Aryeh Leib ha-Levi (1721-1800), also known to history as Hart Lyon,  was the Ashkenazic Chief Rabbi in London between 1757 and 1763. He was  appointed Chief Rabbi of Berlin in 1772. His youngest son 
Zalman  (1761-1842) would achieve fame as Rabbi Solomon Hirschell of London.  Another son, 
Saul  would achieve notoriety as the presumed forger of the 
Besamim   Rosh.
Here is his portrait:
Many thanks to Rabbi Eliezer Brodt, who sent me much  valuable information regarding Rabbi Fleckeles and his attitude toward  the Zohar.
Update: November 22, 2010 - Recently HebrewBooks added an uncensored version from the Chaim Elozor Reich Collection, which you can access here.