The very existence of a Book of Abraham is attested in other sources. Jeff Lindsay explains:
According to Joseph Smith, there was a Book of Abraham. The Bible never mentions this, and many Bible scholars have assumed that Abraham was an illiterate farmer who would not have written a book. There was nothing in Joseph Smith’s information environment to give him the idea that Abraham wrote a book of scripture. Since Joseph Smith’s day, numerous sources have been discovered that point to the existence of recorded writings from Abraham. The previously mentioned Apocalypse of Abraham, the Testament of Abraham, and Jubilees are examples. Many other documents suggest that Abraham kept written records, or that records containing the words of Abraham existed. The Babylonian Talmud calls the book of “Jashar” the “book of Abraham, Isaac, and Jacob” (p. 123). The Muslim writer al-Masudi (died 956 A.D.) wrote that God revealed ten sacred books to Abraham (p. 353). Vettius Valens (A.D. 102-152) wrote a treatise on astrology that mentioned Abraham, referring to what “Abraham showed us in his books about this subject, clarifying the explanations of others and his own, discovering and testing other things, especially concerning the beginnings of journeys abroad. . .” (p. 477). Firmicus Maternus in the fourth century refers to a “tractate excerpted from the books of Abraham” (p. 479).– Jeff Lindsay – Questions about the Book of Abraham, Part 3: Does It Agree with other Ancient Texts?
Where would Joseph Smith have gotten the idea that Abraham wrote a book?