One suggestion on the library, from parents of kids (10yo twins) who reads like yours - sometimes several books a day. For us, the issue is partly the $$ to buy books, but also the space. Even though we buy maybe only 15% of the books read in our house, they still accumulate like crazy.
Our kids only browse comics and graphic novels at the library and won't look at regular books. At the bookstore, they only look at the toys and Klutz books. However, our library has an online catalog where we request books - things we've seen at the bookstore, or found searching on authors/series/subjects the kids are into, etc. We also use Amazon to search for books and read reviews, and then request from the library. Our library even has new book lists once a month and I find some good ones there sometimes. We request the books online and have them sent to our neighborhood branch, and then stop at the library a couple times a week to pick up whatever has come in. We typically have ~50 books checked out at a time. The kids don't read everything we pick out, but we always have new material on hand and sometimes they surprise us by reading something out of their usual zone.
