If you have a Kindle reader for your laptop, then you presumably already have some books which you can transfer to your Kindle.
As well as Nibble's link, look on Amazon. There are some free books, and also some excellent ones for under a pound (unfortunately, there also a lot of utter dross, so it's worth downloading a preview first).
If I'm thinking of buying a book for my Kindle, I always download the preview to my PC first, unless I'm very familiar with the author's work.