Set in the same universe as the Eddings' The Belgariad and The Malloreon, it is effectively one big flashback (or a prequel) to the other series, although the framework story is set after the events of The Malloreon.
The book opens shortly after the end of The Malloreon with Belgarion, with help from Durnik, pestering Belgarath to write an autobiographical account of the events prior to The Belgariad. The core of the book is in the form of Belgarath's memoirs starting with his becoming an outcast from his village and becoming first disciple of the god Aldur and ending with the birth of Belgarion—a span of about seven thousand years.
Belgarath creates inconsistencies with the rest of the books in the series rather than just fill in all the gaps. Although Belgarath explains that the historical texts used previously had inaccuracies, the real inconsistencies are to do with things that contemporary characters who lived through those times had said but have changed in Belgarath. Here is a list of inconsistencies:
According to Belgarath, the name of the Rivan prince who survived the assassination of the royal family was Geran, whereas in Castle of Wizardry, Polgara says that the name was Gared. Polgara, in her story, had an entire section named after Geran. It has been acknowledged that this is due to a typing error in "Castle of Wizardry".
In Magician's Gambit, the Voice of Prophecy tells Garion that he has never spoken to Belgarath directly, whereas Belgarath records many conversations between Belgarath and the voice. Although this may simply stem from the fact that the voice of prophecy explains that it can communicate from any time period it chooses and even asks the party at one point how do they know it isn't speaking to them from a future where the light won.
Also in Magician's Gambit, upon finding the Marag slave woman Taiba who is important to the Prophecy, Belgarath shows very strong relief and states that he has been "worried for a thousand years" about whether she'd been lost or if he could find her. This is contradicted by Belgarath which states he never loses track of where she is.
In Pawn of Prophecy, Belgarath is rather surprised at the use of the term 'Grandfather' by Garion, yet in Belgarath he is used to the term by all of his descendants and acknowledged them all as his grandsons. All of the other descendants knew of their relation to Belgarath from a young age, and thought it natural to call him "grandfather." Garion, however, only found out about his relation to Belgarath at the age of 14, and so his usage of the term so quickly and automatically, with no prompting from anyone, may have been what surprised Belgarath.
In Seeress of Kell, Belgarath and Polgara mention how during the battle of Vo Mimbre the enemy had used illusion in their tactics, and Belgarath recalls trying to keep their side in control, yet this does not actually happen in Belgarath.
In Sorceress of Darshiva, Beldin and Belgarath discuss Beldin's first use of sorcery, discussing how Beldin was angry with Belgarath and did damage to a tree, yet in Belgarath Beldin is the first known example of naturally being able to use the Will and the Word (using it to shock Belgarath by translocating a series of rocks Belgarath wanted to use to build his tower-Belgarath expected him to physically carry the rocks, hence beginning his training via frustration). Although, how to use the Will and the Word does not have to be known prior to using it. So, it may be that Beldin was able to use the Will and the Word naturally, but did not use it for the first time until Belgarath goaded him into it.
It should be noted, however, that the original series (Belgariad and Malloreon) are technically considered to be stories told, whereas "Belgarath the Sorcerer" is the truth as he remembers it, and both are therefore to be considered potentially flawed.