The link below is to an article reporting on how to borrow the Harry Potter ebooks for free – but you do need to be an Amazon Prime member to do so. They are available from the Kindle Owners’ Lending Library.

For more visit:
http://www.mediabistro.com/appnewser/harry-potter-ebooks-now-available-to-borrow-from-kindle_b23908

It would appear that you also need to be in the USA. The Kindle Owners’ Lending Library site provides more information on how to access ebooks.