If the ebooks are of the epub file format and have DRM, you will need Adobe Digital Editions
and an account for it. You'll then need to connect your Mini to your computer while the program is running in order to authorize it or copy books with the Adobe DRM on it to your Mini. (especially if you use a public library - those give you a .ascm file that you have to open with ADE in order to download the book)
If the book has no DRM (such as those from Project Gutenberg
), it's just a matter of copying the file over to your Mini (or going there with the built-in browser and downloading, of course).