I agree that reading isn't seasonal, but book buying probably is.
People tend to stock up around holidays. Before holidays is a good time for print sales, and after holidays for e-books. (Lots of people are giving Kindles as big ticket gifts, and if you receive one, you need to stock it up!)
Right now we're in a stretch of the calendar where there aren't any gift-giving holidays, and people are focused on shopping for back-to-school.
However, reading preferences might be seasonal. I know that in the winter, I tend to read heavier, longer, more dramatic books (perfect for curling up under a pile of blankets), but in the summer, I want short, breezy fun reads (perfect for lounging by the pool or on the beach).