With your Kindle, there are a
lot more places to get free books than just Amazon. A bit like IMSLP, if the author has been dead a certain length of time, works are public domain. Some you may have to manually transfer to your Kindle, but that is easy (plug it in to computer, drag and drop downloaded file in to Kindle drive - done!)
Here is a guide to some places to get free books

I'm gradually working my way through 45+ H G Wells works I got from Project Gutenberg - great stuff

My physical unread bookshelf with actual physical books I have yet to read on it isn't getting any smaller, I still keep buying real books too! But I have got a lot on the Kindle and then don't need physical copies, and it is great on a day like today where I would have to bring two (or more) books with me for my journey but I can just bring my Kindle and have a huge choice of things to read.