If you're looking for a book with decent mechanics, balance, and customization, do not buy the
Encyclopedia of Demons and Devils.
This book is completely out of wack with regards to stats, CR, and overall balance. While I respect the attempt to translate more historically "accurate" representations of malign spirits and personalities, this book does a very poor job of it. I don't have the book here with me to give you some detailed examples, but rest assured that you can find better options.
If you're interested in Demons and Devils, I would strongly recommend:
Legions of Hell and
Armies of the Abyss by Green Ronin. Although neither book is completely dedicated to "historical" accuracy to the degree that EoDD attempts, a significant degree of research was completed for both books (I've included a link to a
review ).
Also, there is WotC's
Book of Vile Darkness, which presents official stats for the various Archfiends (Asmodeus, Demogorgon, et. al) as well as 3ed stats for classic fiends from 1ed and 2ed. Here's another
review .
Good luck!