Read Era is technically free, but I paid for premium years ago and have never regretted it. I can open any kind of uncorrupted book file, from the Amazon reader format to PDF to epub, and everything else I've ever come across. It has a great search function, and the ability to file a book into a custom 'Collection'. You can edit the details of a book, like adding Author or pusblisher info, add your own personal notes to a page or highlighted quote, see an aggregate of all your highlights in a particular file, and adjust the font, background color, and contrast to your hearts content.
I make my whole family use it now, cause I love it so much and Premium works on Family share.