"The Book of Fiends" by various,
published and distributed by Green Ronin.
ISBN = 1-932442-09-X GRR # 1025
Although the Book of Fiends is an older book, I think it is one well worth considering. Here are some details.
There are a total of nine chapters in this volume, which I will look at in four sections.
The first four chapters deal with "The Armies of the Abyss" - a section based on Green Ronin's earlier book, "Armies of the Abyss", but it has been revised and expanded for this book, and has some new material as well. Some of the more important features of this section are the new Thaumaturge class (which I only recommend for NPC's), and the denizens of the Abyss, including some info on the rulers of the Abyss and detailed info on the creatures of the Abyss - namely Demons. There are 21 entries for the rulers of the Abyss, and 43 various creatures of the Abyss.
The next two chapters are concerned with the never before seen "Hordes of Gehenna" and its residents - the Daemons. There are entries for 44 totally new monsters that you can perplex players with. There's also info about the plane of Gehenna, its rulers, and inhabitants.
The next two chapters deal with the "Legions of Hell" - material formerly presented in Green Ronin's "Legions of Hell" book, and once again it has been revised and updated for this volume and includes some new material. There are 51 more residents detailed.
The final section deals with the appendices, including three new Prestige classes (better off left to NPC's). info on Angelic choirs, the most useful section - info on the Unholy Warrior (think "Anti-Paladin"), two fiendish cities, and monsters ranked by challenge rating - all the way from 1/2 to (believe it or not) 40!
There's a useful Table of Contents, =and= an equally useful index. Altogether, there's over 130 various Devils, Daemons, and Demons. The black-and-white art is a bit put-offish, but I think that it is put to good use to illustrate things in this volume. I think this is a very good book for details on the Underworld races and rules. Although 3.5 is a little out of date nowadays, it can be converted easily to other systems. My recommendation to GMs - buy it.