TEBookConverter is a free portable tool for converting ebooks (and other documents) between 25+ common file types.
The program is essentially a shell for Calibre, which means it can handle many different formats. Supported input file types are cbz, cbr, cbc, chm, djvu, docx, epub, fb2, html, htmlz, lit, lrf, mobi, odt, pdf, prc, pdb, pml, rb, rtf, snb, tcr, txt, txtz. Supported output formats are azw3, epub, fb2, oeb, lit, lrf, mobi, htmlz, pdb, pml, rb, pdf, rtf, snb, tcr, txt, txtz.
The program can import individual source files, folders or an entire folder tree, or you can just drag and drop the files you need onto its window.
The output format may be selected from a list, or you can pick a device preset (20+ manufacturers are supported) and TEBookConverter will choose the format itself.
By default TEBookConverter runs up to 8 conversions simultaneously for the best performance, but you can change this to anything from 1 to 16.
Once you've ready, click Start and the program passes your document details to Calibre, which then handles the conversion work.
TEBookConverter didn't run completely smoothly for us. We saw a few error messages, most of which made no sense at all (one suggested the input files no longer existed, but when we changed the output format everything worked just fine).
Still, most of the time TEBookConverter ran our conversions without any difficulty, and as it supports so many file types - and is entirely adware-free - it's certainly worth a try.