I began this project several years ago long before digital books became popular. While I still have several hundred physical books, I've also acquired many digital ones in various formats. This is my digital library.
Most popular format is the 'PDF' but I also have many books in the '.epub' format.
A 'PDF' file is readable from the browser, but an 'EPUB' file is not, without an add-on or extension. In this case the file can simply be downloaded and read later. In fact all titles can be downloaded if you know what you're doing.
You will find a large range of subject matter, but heavy in the STEM areas. No fiction material will be found here, however I have another site Ephemera which you might want to check.