I found the Call of Cthulhu rulebook to be a fascinating read, with reams of info on game play, creatures, spells and the cthulhu mythos. It also contains a whole host of historical data, with the Psychology section standing out in my mind.
This book should be a must for any horror gamer, and any Lovecraft fan should consider it too. My only criticism is the repetition and poor standard of some of the artwork, but the general body of art is still provocative and inspiring.
A solid, professional piece of work.