Sauce hit most of the must reads on my list, but if you can get a hold of it, Nick Lenz's Bonsai from the Wild, is a great book if you are interested in native American species. And from the other end, I found Amy Liang's The Living Art of Bonsai to be very good too, her styling is a mix of traditional Japanese and Chinese (penjing).
But really, any book is better than no book
