You mean Belgarath the Sorcerer (BtS)? Yes, it is written in the same manner as PtS in that they are both prequels to the Bel-Mal series. So, if you liked old Belgarath and want to know more about his history, you can't miss this book.
The Rivan Codex is a much much duller read though. If you like all the back histories, cultures and such, it is good.