The g'mara on Pesachim 62b says (Soncino translation):
Rami the son of Rab Judah said: Since the day that the Book of Genealogies was hidden, the strength of the Sages has been impaired and the light of their eyes has been dimmed.
A speculative, unsigned note says that this probably means the book was either suppressed or forgotten, perhaps destroyed. But it doesn't say what it is, nor could I find any references to it on this site.
What is -- or, I suppose, was -- the Book of Genealogies? Chazal must have known something about it since they referred to it in the talmud. Rashi there says (according to Soncino) that it contained the reasons for many scriptural laws that have been forgotten. Was this a rabbinic writing? A text of divine origin?