The library of Prague's Chief Rabbi David Oppenheim (1664-1736) is famous. He is regarded as the first real collector of Jewish books, amassing about 6000 seforim and 100 manuscripts. His collection was so complete and so impressive that it formed the basis for one of the landmark books in the history of Jewish bibliography, Johannes Christoff Wolf's 4-volume, 5000 page Bibliotheca Hebraea (1715-1733). Incidentally, if it seems odd for an אב"ד to allow his seforim to be examined and cataloged by a Christian Hebraist for a major bibliography written in Latin and aimed at the wider scholarly world, one considers that the oddity is with thinking it odd.
The details regarding this library, the fact that R. Oppenheim was unable to keep it in Prague with him due to fear for the safety of his most valuable collection, can be read elsewhere. However, where it ended up and how it got there is most interesting. After he died, his library was kept in crates, in the home of a relative. It was offered for sale, with a catalog created for that purpose in 1764, but for decades no interested buyer purchased it. It's value was presumed to be high. Moses Mendelssohn estimated it should fetch 60,000 Thaler. Later, it was estimated to be worth over twice that. Ultimately, the Bodleian Library at Oxford made what has to be considered one of the best deals of all time (at least for a bibliophile) and bought it for 9000 Thaler, in 1829. This was the equivalent of £1350 (a rough search on the internet reveals that this was equivalent to about £95,000 pounds in 2007. A very, very good price. Ultimately, this library formed the core of the Bodleian's great Hebrew collection.
Solomon Schechter reminisced about the rumors surrounding this famed collection of Hebrew books in his native country:
Prior to the sale, it was cataloged once again (in 1826, by Isaac Metz) and printed as Collectio Davidis, or קהלת דוד. This fascinating book is now available online (link). You can also view a digital version of one of Oppenheim's manuscripts here (Ms. Opp. 154; a 15th century work called by its author משל הקדמוני.)