Odd things: Sometimes quite difficult to persuade it to find books on, for instance, the Amazon.co.uk catalogue, even when you can find said books yourself on said catalogue (most recent such example: my 1954 Regent Classics edition of Moby-Dick). For some of my other books it's understandable that the search robots couldn't locate them, either on grounds of geographical origin or age.
I'm interested that apart from genre literature the next most popular categopry appears to be theology, with top non-fiction author being John Piper. The only other author of the current top 25 who I simply haven't heard of is Tamora Pierce. I have at least read all the others apart from Mercedes Lackey. Of the top 25 books I've read everything except The Catcher in the Rye, A Heartbreaking Work of Staggering Genius and Life of Pi. But my list of unread books that I already own is quite long enough.