hi! there are some biblical historians here, but fyi, this question might be worth also x-posting to /r/AcademicBiblical

Several "lost books" were included in the Ethiopian Orthodox Church's Bible. Additionally if you are a mormon, there are other "Lost Books" of the bible. There are even different cannons for Protestant and Catholic bibles as well. The term "lost book" for excluded book is not an overly good descriptor. Many of these "lost books" were excluded in one place, and included in another. Apocrypha from early Christianity was widespread, and lots of it was written. Some dealt with the life of mary, others Jesus' early life, and still others dealt with acts of the apostles. Fragments and manuscripts of these books have survived. EDIT i agree with /u/searocksandtrees /R/AcademicBiblical is a better place.