MobileRead has some great free books with improved formatting and forums about free ebooks formatted for various readers.
https://www.mobileread.com/forums/ebo...&sort=dateline
https://www.mobileread.com/forums/forumdisplay.php?f=132
If you don't like book cover that comes with a free book, Calibre might find a better cover available for download using edit metadata function. Right click on book listed in your Calibre library brings up this function.