Hi Danny. First, let me say that I am a fan of your reviews and have found them informative and helpful in the past. So, I am very interested in seeing your site rank highly in Google. The information is the type of information I want to see when I search for reviews on books that I am considering reading.
My best guess, aside from the index problem, is that you are running into a nebulous user-experience issue. Your site has been around for so long that the design is now out of date. It looks like a legacy site that is just running on the laurels it received 10 years ago. I'd suggest updating the design to a more modern look and adding in some web 2.0 stuff. Let people comment on your reviews--moderate the spam out, of course. Add some social networking functions to drive visitors that way too.
Basically, I suggest making it look more "modern." You also need to file DMCA notices against those people who are stealing your content. The other issue is that a lot of book reviewers cross-post their reviews on Amazon. I don't know if you are doing that, but stop it, if you are.