I read today an essay by Professor Shmuel Krauss z"l about "Sefer Yochasin" (the Book of Genealogies). He theorized that traditions such as the name of David's mother, the identification of Boaz with Ivtzan and many many more such traditions in the Talmud, Midrash and even some Targums may have been part of Sefer Yochasin, which was a baraita to explain and expound upon the genealogies of Divrei Hayamim (per Rashi).
In Pesachim 62b it says:
"Rami bar Rav Yuda said that Rav said the following about it: From the day the Book of Genealogies was hidden and no longer available to the Sages, the strength of the Sages has been weakened, and the light of their eyes has been dimmed, as the book contained the reasons for many Torah laws and lists of genealogies that are now lost."
My question is why was it hidden away? It seems like it contained important information, especially if Krauss's theory is to be accepted - bits of this information were certainly important enough to be included in the gemara! Especially as the gemara explains in Bava Batra 91a that this information was used when arguing with the minim about traditions of Judaism. If it was lost, why not write "מיום שאבד ספר יוחסין" ("from the day that the Book of Genealogies was lost...")?