I think that The Rivan Codex is a bit unnecessary book. The best way to describe this book is to say that it's a history book for the world that Eddings has created (it's a companion book for The Belgariad, The Malloreon, Belgarath the Sorcerer and Polgara the Sorceress).
This book may offer some interesting information for die-hard Eddings fans, but other people shouldn't read it.