The ASA book is good but the Gleim book is far superior. (Amazon didn't carry the Gleim 08 version when I needed it so I had to go directly to the Gleim site). Both books are laid out similarly: major study sections with several introductory overviews of main points, followed by multiple-choice questions from the current FAA test.
The Gleim book is far superior for one simple reason: Gleim explains why all of the incorrect choices are wrong. The ASA book rarely does. I found the Gleim book invaluable in my study for the written exam (93%, thanks to Gleim), even though I bought both the ASA and the Gleim books. In fact, the explanation of a wrong answer was often more illuminating than knowing I'd chosen the right answer, because it made me think and make better mental connections.
Gleim even emailed me back within 12 hours when I had a question about some of the airspace material that I found contradictory. Their explanation was very thorough and professional. When I emailed to thank them for helping me achieve a successful score, they emailed back and thanked me for the compliment.
I'm an instructional designer by profession and I can say without reservation that Gleim is the better choice.