The Book of Sith was a collection of Sith historical writings assembled into a single book by Darth Sidious. It was bound sometime shortly after the Sith Grand Plan succeeded at dismantling the Galactic Republic, exterminating the Jedi Order, and in placing Sidious in undisputed control of the known galaxy.
The book assembled a group of journalistic writings by various dark siders spanning from writings by Palpatine himself, all the way back to the Second Great Schism nearly 7,000 years prior. Besides Sidious, authors included Sorzus Syn of the old Sith Empire, Darth Malgus of the reconstituted Sith Empire, founder of the Order of the Sith Lords Darth Bane, Nightsister leader Mother Talzin, and Sidious' own master, Darth Plagueis.
As a result of Operation: Knightfall, which saw the Jedi Temple's occupants and stewards executed as traitors to the Republic, the Archives were opened to Sidious' exploration. It was in the ruins of the Temple that he uncovered these ancient documents, and compiled them into a single volume.
Each of the documents has margin notes hand-written by the various people who held them over time. These include Yoda, Mace Windu, Quinlan Vos, Asajj Ventress, Darth Vader, and Darth Sidious, as well as newer annotations penned when the book was rediscovered by Luke Skywalker in 24 ABY.
Rather than turn it over to the authorities when he discovered it, Skywalker took the book into his own possession. He feared it may have contained dangerous secrets which could not be made public, and kept it for study before deciding on its fate.
Sorzus Syn's chronicle
Darth Malgus's war journal
During the Outer Rim campaigns in the opening years of the Great Galactic War, Darth Malgus kept a journal to record his thoughts and experiences. These pages recall some of his struggles in the first years of the war.
The Rule of Two
Some time after the end of the New Sith Wars and Darth Bane's foundation of the Rule of Two, he collected philosophies that would guide the Sith Order for the next thousand years, entitling his work The Rule of Two.
Scientific journal of Darth Plagueis
A scientific journal by Darth Plagueis with notes about his experimentation with and manipulation of the Force.