Described by Thoth as a field study, the book, known simply as the Book of Thoth was written by Thoth when he was younger. It contains many spells and information about the gods that is dangerous in the wrong hands, including the locations of their palaces, all of the forms they can take on and where all of their sheuts are located and how to summon the gods and their sheuts. It also has a map to the Land of Demons though its very hard to understand according to Zia Rashid. Apparently the map serves a combination compass, tourist's guide and Farmer's Almanac timetable. Presumably it also contains information on the other areas of the Duat. Setne added spells for capturing and destroying sheuts after he stole the book and it has a spell that is capable of imprisoning gods.
In Egyptian Mythology, the book contained all the knowledge of the universe.
Setne (portrayed as a hero in mythology, unlike in the novels) stole it from the tomb of the previous person who stole it, Prince Naneferkaptah. The ghost of Prince Naneferkaptah himself told Setne how Thoth cursed him by killing his wife and child, and told him to leave the book. Setni ignored the warnings, and took the book, prompting Prince Naneferkaptah to curse him.
Afterwards, he was seduced by a beautiful woman into deserting wife and killing children, and then discovered the beautiful woman to be a corpse. Luckily for him, it was a dream sent by Prince Naneferkaptah to warn him of what he would do to Setne if he did not return the book and bring the bodies of his wife and child to his tomb to dwell with him. Naturally, Setne agreed to do this and did as he was asked.
This book was stolen by Setne when he was alive and hidden, but before he died, he used the information in it to create a number of spells, including a shadow execration which is capable of destroying a god. That information he put into the Book of Overcoming Apophis and with that destroyed, the Kanes work with him to retrieve this book to learn how to destroy Apophis.
They succeed in retrieving it from where Setne had hidden it behind many traps and an Apis Bull and use it to track down and summon Apophis' sheut. Setne then betrays them and steals the book as he needed them to summon the sheut because as a ghost he can't cast even a simple summoning spell. However, he can cast the binding spell and plans to use the sheut blackmail Apophis into working for him. Sadie Kane arrives and rescues Carter and Zia, but when an army of demons arrives, Setne gets away with the book.
Carter and Sadie are able to use what they learned to destroy Apophis and Setne later comes to congratulate Carter and is seen with the book which he gets away with again. Setne now has the potential to cause great trouble with the book, though as a ghost he lacks the ability to cast most of the spells.
Setne is revealed to have been using the Book of Thoth to combine Greek and Egyptian magic by summoning a Son of Sobek and Serapis. However, while Carter, Sadie, Percy Jackson and Annabeth Chase manage to stop these plans, they realize they are likely just tests.
Setne attempts to use the Book of Thoth to gain immortality by getting the Crown of Ptolemy from Wadjet and Nekhbet. Carter, Sadie, Percy and Annabeth team up together to stop him. While they fail in saving Wadjet and she is consumed, they are able to save Nekhbet and Carter severs Setne's hand and retrieves the Book of Thoth. Believing he doesn't need it anymore, Setne lets them take it as he gets the Crown. Trying to figure out a way to defeat Setne, Sadie suggests using the Book of Thoth since it helped them destroy Apophis. Nekhbet reveals that the Book contains a spell to imprison gods, but that it would require a combination of Greek and Egyptian magic, an enchanted circle and rope, consecrated ground and other things that would have to be improvised. In order to perform the spell, Sadie reveals her secret name to Annabeth, allowing her to use Egyptian magic and help her create the circle. Carter enchants the rope while Percy hosts Nekhbet to fight Setne. Using a combination of his own power over the sea and Nekhbet's abilities, Percy defeats Setne and brings him to Governor's Island where the others have prepared the spell. There, he throws Setne into the prepared circle. After removing Riptide from Setne and releasing all the magic he has stolen, including Wadjet, the four cast the spell, binding Setne in the rope as part of it. Setne ends up imprisoned in a plastic snow globe and Carter and Sadie take the Book of Thoth with them when they leave.