On a book's landing page on Leanpub, check to the left of the book cover. You might find a "Sample Book" there that you can view.

Please note that we have a 60-day refund policy, so you can buy the book and if it doesn't meet your needs, you can get a refund:

http://help.leanpub.com/reader-help/how-do-i-get-a-refund