Investing time in reading is great value - if the book you're reading is going to help you grow and develop. You've already paid for the books in your local library, so if you're looking for a book to read, you should check out the library first.
But let's say you need to buy a book. Then what? The more prices and sources you can find, the better value you can get.
I've created a tool for comparison shopping for books before you buy. You can find it here.
If you're just looking for any book and are allergic to libraries, then visit your local Thrift Store. These are great sources of used books.