On February 26th, Kindle customers will lose the ability to download eBook purchases directly to their PC. If you want to switch to a rival eReader brand in the future, I suggest that you use the soon-to-be discontinued “Download and Transfer via USB” feature to archive your Kindle library.
My favorite sites for actual ebooks are Humble Bundle and Fantastic. But these are predominantly tech books. No idea where I’d get good fiction in epub today.