This is an ancient work that existed before the Roman Empire became Christian. It claims to include the prophecies of Eldad and Medad. These were two elders whom Moses appointed while the Israelites were in the wilderness (Numbers 11:26). The Bible says they prophesied, but it does not tell what they said.
Because of this gap, someone wrote a separate work about them. That book has not survived. The only known quotation from it appears in the Shepherd of Hermas: “The Lord is near to those who return to Him, as written in Eldad and Medad, who prophesied to the people in the wilderness” (Vision II, chapter 3). This is the only surviving sentence that tells us anything about the content of the book.