Books are not free once you buy a Kindle. You still have to buy the ebooks, although there are a good number of free and very low cost ebooks at Amazon (mainly ebooks written by new authors or independent authors).
BUT - ebook bestsellers are often NOT cheap and some publishers are very greedy and charge more for their ebooks than the cost of paperbacks.
SO - the cost of ereading depends on what you want to read. Once you buy the Kindle, the cost of reading for various ebook types...
1. Old Classics (like Little Women) - mostly free
2. New ebooks by some indie authors - some are free & many are very cheap
3. Ebooks by established popular indie authors - reasonably priced, usually less than the cost of an average paperback and often under $5
4. Bestsellers, hot popular reads, books by famous authors published by real publishers - usually not cheaper than a paperback and may cost as much as a new hardcover.
I read cheap - I read mainly indie ebooks that I get cheap or free on Amazon. And occasionally I spend up to $10.99 to buy a bestseller or famous author ebook that I really want to read.
Are you in the UK? If so, here is the UK version my favorite site that points out the latest free ebooks at Amazon UK. This is VERY helpful to catch new indie ebooks that are briefly free.
http://www.dailyfreebooks.co.uk/
And at Amazon UK, from the main Kindle Ebooks page, pick Best Sellers then pick Top 100 Free. Then you can see the top free ebooks and also pick categories on the left to see the Top 100 Free in a category. Here is the Top 100 Free list at Amazon UK
http://www.amazon.co.uk/Best-Sellers-Kindle-Store-eBooks/zgbs/digital-text/341689031/ref=zg_bs_fvp_p_f_341689031?_encoding=UTF8&tf=1