: A popular series for learning Polish as a foreign language.

: Sites like CloudConvert or Zamzar allow you to upload a PDF or EPUB and download the FB2 version directly. Rethinking Ukrainian History - Internet Archive

If you find a book you need in a different format (like PDF or EPUB), you can use free tools to convert it to :

: Primarily hosts older, public domain literature. While it may not have modern textbooks, it is an excellent source for classic Polish literature in various ebook formats including EPUB and MOBI, which can be easily converted to FB2.