You could try gutenberg.org, which tends to have rather old stuff (right up my street).
gutenberg.au, the Ozzie site has some modern stuff.
Best two for me are feedbooks.com and alfalib.com, although the latter has some weird formats, and sometimes no download method for particular titles.
Alfalib has changed its method - you can no longer download direct; it passes you on to Amazon and other sites, with their regular charges.
(To be fair, Amazon do have some free rmodern content, e.g. Harlan Coben's The Woods)