1. read them free from the library. If your library doesn't have it recommend it for them to buy or get an interlibrary loan.
2. garage sales, Goodwill, Salvation Army, used book stores
3. tip: Children's books at Goodwill are $.38. If I find one in good condition that I don't need I still buy it. Then I turn it in for store credit at a used book store. On a $4.00 book that I bought for $.38 I get $1.00 in store credit. I actually come out ahead.

Library book sale - yearly event here.

My new favorite is paperbackswap.com
I list books online, others can request my books. I send it out to them, when they receive I can pick a book from some elses bookshelf anywhere on the network and they send to me. The cost, it the price of postage when you send a book off. Costs nothing to get a book as the other person will pay the postage to get it to you.
List 9 books and get 3 credits to request a book right away. I've gotten rid of books I'm done with and picked up some great news ones for my bookshelf.

I joined paperbackswap.com over a year ago myself. It is a great site, but i ended up clearing my bookshelf off because i just have too many books here at home. I still like to visit the site for book recommendations,etc.

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"Go on, get outside, get the stink blown off ya!!"

Barnes & Noble has a summer reading program for kids, read 8 books, get 1 free with a max 4 books/kid...they have some really good classics.

I get my books from the library! If they don't have it. . .which is most of the time since it is a small town library, I get them through inter-library loan. Just read a cool one called CheapTalk with Frugal Friends (at least I think that was the name.)