The Book of the Dead is a funerary text that was used in Ancient Egypt. While on the first mission to Chulak, Dr. Daniel Jackson commented on the existence of Ra and Apophis as something out of the Book of the Dead—essentially, the Goa'uld were living it. (SG1: "Children of the Gods")
The original Book of the Dead had a much darker history among the Goa'uld and was not, in reality, left on Earth. The book was possessed by Sokar which contained a list of all the people that were interred in Necropolis. It also had notes on scientific discoveries such as deadly poisons and portable particle cannons.
Jaffa myth stated that if Sokar wrote a persons name on this tome then they would die within a week's time. Tok'ra scientists believed that the Book of the Dead was actually device capable of producing poisons and splicing it into another being's DNA. This allowed the System Lord to kill any person by using the book without ever being detected. After Sokar's death, the books whereabouts were unknown.