After having read this book cover to cover, I thought I'd give some feedback on it. There have been several updates from the previous book on demons and devils, The Book of Vile Darkness, which I will come to later.
Fiendish Codex II: Tyrants of the Nine Hells is a nice counterpoint to the previous book in the series, Fiendish Codex I: Hordes of the Abyss. It is laid out in the same fashion as its predicessor, with an introduction that mainly consists of a fairly long story about the origin of devilkind. This includes the history of Asmodeus and how he tricked the lawful dieties into signing the contract full of loopholes that handed him the authority to handle punishment of souls, and indirectly subvert and trap them.
The first chapter of the book mainly deals in generalities about devils with sections on what motivates them, how to use them as more than just throwaway encounters, Faustian Pacts, the Blood War, biology, and a condensed recent history of the Nine Hells, including the Reconing and the current political climate.
The second chapter goes into the details of each layer, including the population breakdown, which unique devils and lawful evil dieties populate the level and what each diety's relationship with the Arch-Devils is, as well as major cities and landmarks. There is a brief introduction to the Arch-Duke/Dutchess of the layer, and tips on roleplaying them for the DM. Each section closes with two or three encounters, most of which I was pleasantly surprised to find are quite in depth.
The third chapter is pretty short and is basically monster type bios and stat sheets, as could be expected, almost all of them are devils. Most of them are new, but there are some carry overs from The Book of Vile Darkness.
The fourth chapter introduces a new race, a small number of new classes, some spells, traits, and advantages and disadvantages on "selling your character's soul to the Arch-Devil" so to speak. The hellbred are souls that repented their evil just as they died, and so to find out which way they would swing, they are then reincarnated by the gods as a hellbred to decide their fate. Only a truely heroic feat or sacrifice can save a hellbred from an eternity of punishment. The new classes are as follows: a thief that focuses on stealing from devils, a cleric-type that focuses on saving the souls that are already trapped in Baator, a fanatical paladin type that always wants to be on the front lines, and a sorceror type that handles the Hellfire that Mephistopheles has been working on. Spells I merely glanced over, but most of them seem to focus on giving the target aspects of a specific devil type for a short amount of time. Traits are mostly carried over from The Book of Vile Darkness, things such as Dark Speech, monsterous scales, devilish vision, and so on. Out of this section, I found the advantages/disadvanges granted by the Arch-Dukes/Dutchesses the most interesting. There is a detailed write up for each, and how they are tied to the devil sponsor. If you're running an evil campaign, you'll definitely want to give this section a look.
The last chapter is devoted to the Arch-Devils themselves. They are arranged by layer, and all of them are given a biography in addition to the ones detailed in their layer description. There are also notes on their cult in the Material Plane, who is who's enemy and ally, how these relationships have changed since The Book of Vile Darkness, and how to leverage them for the PC's benefit. The stats given are for the aspects of the Arch-Devils, however.
Why I knocked off a star: The main thing that bothered me was that there is almost no information given on the unique devils that are between the pit fiends and Arch-Devils on the heirarchy. Just names at most. More detail in this respect would have been appreciated.
The second thing is that the stats given for the Arch-Devils are for their aspects, not for the Arch-Devil themselves. Now, this is probably not a problem for meeting them on the Material Plane or other places except for Baator. If you have ultra high level PCs going to Baator to fight them directly, this could be a problem because an aspect cannot manifest on the same plane as the original. They should have included a double stat sheet for the Arch-Devils, one for the aspect and one for the actual devil. You can use the stat sheets from The Book of Vile Darkness, but it would have been nice to have them all in one place.
The last few things are more nitpicks, really. Personally, I find the tale in the introduction a bit farfetched, considering that you'd think the gods would actually read what they were agreeing to and would be smart enough to recognize that Asmodeus was putting one over on them. Some of the art has been recycled from The Book of Vile Darkness, and as previous reviewers have noticed, not all of them are correct. There are also a few minor retcons and omissions from the information in previous books. For example, the origin of the nippurbio has been changed, and there is no mention of the yearly meeting between the Arch-Devils to report on the Blood War to Asmodeus.
Overall, this is a good book to have if you're running a campaign with devils. It takes almost everything from The Book of Vile Darkness and some of the previous, more general book on demons and devils and expands on it. I was particularly interested in the political shakeup and changing relationships between the Arch-Devils. There is a lot of information between these covers, and any DM with even a passing interest in running a campaign with them will find something useful.