Books can become an expensive habit, but you need to have a lot around, and get new ones regularly or even an avid reader will eventually tire of the same stories. The first and cheapest solution is, of course,
Go to the library, hand out there to read or take the books home. In the hot summer days, libraries are particularly nice places to hang out. Cool and quiet, many of them offer activities such as readings, tutoring, and classes, as well as programs to distribute free books to take home and keep.
Second excellent solution to the cost of books is buying them second hand
I think the idea of a spanking new book is over-rated. As long as the book doesn't smell of mildew and the pages hold together, who cares?
Ebay or Craisgslist won't save you much money, because of shipping charges or the hassle of picking up a single volume from across town. Church sales, stoop sales, thrift shops, and friends with older children are great sources for books in bulk. In my neighborhood lots of people also offer books for free: they put a box on the stoop with a "take me!" sign for walkers-by. I live in an apartment building with a common laundry in the basement. Next to the washing machines there's a bookcase which serves as a free book-exchange. Residents leave books for their neighbors, and take them at will. If you don't have an informal "library" like this in your building you might be able to set one up.