IMPORTANT: the following instructions are only applicable for books in the EPUB format.
Apple Books is a wonderful e-reader application that comes with all MacOS computers and laptops. As of this writing, there are no pre-packaged e-reader applications on Windows devices. However, Prismatext recommends both Calibre and Thorium, which are both cross-platform and open-source e-reader applications.
Additionally, it is possible to read a book directly in a browser: