As an avid reader of O'Reilly titles, and as a Windows-based technical support specialist and consultant, I was pleasently surprised with many aspects of this book.
The book's relatively plain, clear descriptions of the inner workings of the Windows filesystem and registry were insightful yet did not deter the reader with dry textbook-esque prose. A full compliment to Mr. Farquhar for his no-nonsense, down-to-earth approach to this subject, geared towards enthusiasts yet appropriate for even a basic user's level of need and knowledge.
Some more arcane, unusual solutions to various bottlenecks in both the PC and Windows architectures were a bonus, worth the price of the book alone. Also, a much more opinionated view of the subject is presented here, which is a relief - many such books tread very lightly around such situations as recommending one product above another.
One addition I would have been willing to pay for would have been a CD-ROM or disk with all the appropriate freeware/shareware utilities mentioned in the book on it. While the concept of "shovelware" is something I don't like, the cost of a CD-ROM is negligable and the hassles involved in digging up the utilities online definitely negate an additional $5 cost in the book, IMHO.
Other improvements would be more comprehensive information on dual-boot scenarios, coverage of further dual-boot utilities (OS/2 boot manager comes to mind), optimization of print applications/spooling, and perhaps a broader range of information concerning registry backup and optimization under Windows 98.
Overall, this is a very good book for the money, as most of your books are, and I recommend it to anyone seeking to learn more about how to improve their Windows experience, or involved in a highly Windows-centric work environment. Will there ever be a Windows NT version of this book?

- Robert