As a Subaru junkie, I have to buy books like MRT's “Training WRX” to fill my need. So I'll get the worst thing about it out of the way first: it's 40 bucks, American, plus $6 shipping. Ouch. But, it gets worse: the book has cheap-o spiral binding, and advertisements on the back. I'm all for capitalism, but a book that expensive should at least be a bit higher quality and free of MoTeC ads.
I ordered it from MRT way down under in Australia, and it was in my mailbox a week later. Some U.S. companies should take lessons from them!
The book gives a very complete overview of what you can do to a Subaru Impreza to make it a street-legal monster. Chapters are divided up by topic like intake, exhaust, brakes, suspension, etc. Within each chapter, the author goes through and explains the cost/benefit and drawbacks from all the known modifications to that system. E.g., in the chapter on brakes, he talks about pad upgrades, STi/Brembo brake kits, brake fluids, etc. It seems most useful when he goes through the drawbacks of various systems. For example, he talks about how you can “cut corners” to accomplish various tasks (e.g. using the O2 sensor to keep tabs on the air/fuel ratio) but that the information you get is very limited in certain circumstances (e.g., the sensor doesn't work well at high rpm, etc.)
What makes this book useful is that it is extremely complete and comprehensive. The introduction is written by a guy with an Aussie WRX who had never modified his own car before until he bought a Soob. This is a handy perspective, particularly since I'm in the same boat. There's information here I have never seen on the web, and it's presented concisely.
The book had a few obscure Australianisms like “pub car park cred”, but not as many as those lame Crocodile Dundee movies made me expect. The book gives prices for all the upgrades in Australian dollars, so have your conversion handy ($1 Aus = 65 US cents).
For me, it was worth it, but I'd feel more comfortable recommending it if the price were a bit more sane ($25 is more like it).