I found this really cool website where librarians suggests books for kids to read by grade, genre, and favorites. So helpful for summer reading! I've already found a ton of books to get for my kids for our next stop to the library.
You can order a box of books from this site. They will range from toddler to young adult books. Each box contains approximately 55 lbs (give or take). You can also order more age/grade specific books, but it costs a bit more.