It's still technically in beta stage, but I've been impressed with Microsoft's Live Book search program, or as they call it "Live Search Books." (I know the primary brand is "Live Search" and their intent is to tack on all formats after that, but "Live Search Books" sounds terribly awkward to me...)
Naming conventions aside, the service's results and usability are solid. Here are the results for a search of the term "blogging." If you hover your mouse over any of the titles in the left pane, the right pane changes to show the results from that title. Those title results also include a colored bar showing search relevance throughout the book -- the darker the green the more relevant that section of the book.
I clicked on our own Naked Conversations and this screen was displayed. Individual page links are ranked by relevance in a scrollable area on the left and the book pages themselves are shown on the right. Notice that the relevancy bar is now displayed vertically; dragging it over the darkest green area allows you to jump directly to that passage in the book. OK, searching for "blogging" in a book about blogs isn't a great test. I read this book though and I recall the authors mentioned something about Lotus founder Mitch Kapor. Here's a search for his name; as you can see, there were two references to him in the book and they're easily found via Live Search.
Live Search renders each page quickly and the results are crisp and clear. You can't read the entire book using Live Search but you should be able to determine whether it contains the coverage you're looking for. For example, after doing a handful of searches within Naked Conversations I still had about 40 pages of results I could read before I'd be locked out of that title. That seems sufficient to me if I'm really just doing some research and either looking for a book to buy or a quick answer to a question.
Microsoft is in the process of adding books to this service and I'm not sure when they plan to take it out of beta stage. The functionality is solid today though so stop by and take it for a test drive.