By Dream Pod 9,
The D20 Mecha Compendium is a book of big robots, mechs, and similar creations for use in D20 games. Produced by Dreampod 9, who has a long history of providing quality mecha related products, the book is 160 pages of stats and background material. The layout is excellent, as I've come to expect from DP9, with plenty of manga style illustrations (though some are skimmed from earlier products).
The book is split into four chapters, the first covers the basics of mecha in D20. The other three cover settings and stats for fantasy, modern, or futuristic games. Each of the setting sections includes a brief overview of a setting, followed by some stat blocks for the mecha in question. There's also advice on customizing the ideas to your own setting, and using them outside the genre suggested.
The first chapter skims through the way mecha work, and lets us know that full details are available in the Guardians of Order product D20 Mecha. That said, enough rules are given that you can run a basic mecha campaign with just this book, you just have a limited number of vehicle modifications. Since the book is based solely around pre-designed systems, I don't see this as a particular issue, but you will probably need the other book to create your own machines of doom.
Chapter 2 is about mecha in fantasy games. It gives advice on why you might want to use them, and some sample settings. The first setting has the mecha as divine artifacts representing aspects of the primal forces (fire, war and so on), with lesser mecha being craftsman created servants of the divine. Quite fun, though not really enough information to run a campaign in the world. The next setting details golemsuits, mecha created like golems. These are pretty dull really, and have the least inspiring illustrations in the book. The beast lords follow this trend, being animalistic mecha in a fantasy world. The gem of the fantasy chapter are the coalsuits of the Cities and Empire setting, which have a nice steampunk flavour.
In chapter 3 we get modern and alternate history mecha. A chance for DP9 to do D20 stats for their Gear Krieg game. Gear Krieg is a nice pulp-ish setting, and the mechs have a suitably retro feel to them. The setting info is sparse, but enjoyable. In the Phoenix Rising setting that follows we get some Metal Gear Solid style mechs. This is a nice little overview of a slightly into the future world with global corporations and power-suited terrorists. I'd have liked to see more detail on it. Great Machine Decander rounds off the modern settings, and is a modern setting where discoveries of a lost civilization have lead to advances in technology.
The last chapter is about future worlds, and includes the DP9 Jovian Chronicles and Heavy Gear setting, along with a few others. The Terra Nova setting for Heavy Gear gets a solid introduction, and may inspire readers to delve deeper. I've always liked this setting, and the overview gives a good feel for the political climate. The Jovian Chronicles is a solar system based campaign with large flying mecha suits, less gritty than Heavy Gear. The chapter is rounded off by a two earth vs the aliens mecha settings, one of which has a passing similarity to robotech.
Overall: If you want to move your Heavy gear, Grear Krieg or Jovian Chronicles game to D20, then this book has the basics of what you'll need. If you want some stat blocks and pretty mecha pictures for a fantasy or modern game, then it may help too. Unfortunately background information is too sparse for the book to be really useful. More a setting taster book than anything else, I'd have preferred fewer settings with more depth of coverage.