Manual of the Planes discusses the other planes of the Dungeons & Dragons world -- the areas of reality beyond the mortal world. Its goal is to allow for adventures set among these other realms -- primarily, the Shadowfell, the Feywild, the Elemental Chaos, and the Astral Sea. It mainly succeeds.
The first chapter of the book, Exploring the Planes, deals mainly with traveling to the planes and the characteristics of the planes. It includes a description of the basic cosmology of the D&D world, some advice for creating alternate cosmologies if you should desire to, and some notes about Sigil, the City of Doors, a location which can be used as a center for planar adventures (among other things).
Those who've played Planescape in earlier editions will recognize Sigil, and it's only one of many references to previous editions of the game. Veterans will notice new treatments of such things as the City of Brass, the Isle of Dread, the Demonweb, the Blood War, and spelljammers. Newer players need not worry; the book sets these elements adequately within 4e, so that no previous experience with them is necessary.
The next four chapters deal with the major planes suitable for adventuring: The Feywild, domain of faerie and preternatural wilderness; the Shadowfell, decayed echo of the mortal world shrouded in gloom; the Elemental Chaos, home to such locations as the City of Brass and the Abyss; and the Astral Sea, in which the domains of the great powers float like islands. Each of these chapters has four sections: traveling to the plane, exploring the plane, sample inhabitants of the plane, and sample locations within the plane.
The writeups about the inhabitants and locations are fairly brief. They're sufficient to give the flavor of the place, and hopefully to inspire a GM to flesh them out and adapt them to his game. They are not, however, fully detailed writeups that are able to simply be dropped right into the game. The book leaves much of the design work to the individual GM. Some people might consider this a weak point; I consider it a strength, because it fits my preferred style.
Chapters six and seven introduce more mechanics. Six is a collection of monsters from the planes for the GM to employ -- a little over a dozen of them, of which half are demons or devils. These are useful as adversaries, but there's nothing outstanding here. Still, nice to have.
Chapter seven is for players -- new paragon paths, rituals, and magic items. Here, too, there's not much that makes me pause and double-take, but nothing that leaps out at me as broken, either. It should serve its purpose well enough.
Overall, I'd say the usefulness of this book will depend on the GM using it -- and it is a book for GMs, with little that's of interest to players.
I think that it's the sort of book that will inspire me, even if I use relatively little of its content as-printed in my game. On the other hand, I wish that there was more: more about the Far Realms, more about some of those locations, more about the anomalous realms like the Plane of Mirrors and the Plane of Dreams. It's probably unfair to expect a 160-page book to cover all of those planes in depth, especially when I'd probably end up cheerfully ignoring most of that depth anyway and using my own creations, personally. Even so, it just feels too brief at times.
I like it, though. I like it a lot. This is a product that makes good use of the history of the game and its lore, without becoming a slave to it. It puts new twists on old friends like the Isle of Dread and the City of Doors, and it fits things like spelljammers into 4e without also bringing along the annoying parts like helm-related bookkeeping, phlogiston, and gravity wells. I've been satisfied with supplements that have done much less.