The only book I currently have is Walstad's.
When I look in a library or in a bookstore, it seems like the vast majority of books are over-simplified start-up guides (fish need water), or overly specific guides to species and such. I don't see many good mid-range skill level books. In the past I've come across some that I really liked, but I can't remember the titles, and I think they were usually translations of European books. Stuff like setting up biotopes and aquascaping tips/techniques. I've learned more from the old alt.aquaria usenet postings and a handful of websites then I have from books.