I still mostly rely on Project Gutenberg and its partner sites like Faded Page for anything published before 1928. The selection's massive for classic literature, philosophy, and early scientific works—everything's public domain, so you're completely in the clear. The formats are straightforward, usually EPUB or plain text, which works on any e-reader.
For newer stuff, the legal landscape gets trickier. I’ve found that some authors and small publishers use platforms like Smashwords to offer permanent free downloads of specific titles as a promotion. It’s not a 'library' in the traditional sense, but you can filter for $0.00 and legally keep those files. My local public library's OverDrive system is the real MVP, though. It requires a library card, but the digital borrowing model is the most legitimate way to access contemporary ebooks without cost.
The distinction between 'free forever' and 'borrowed' downloads matters. Open Library's controlled digital lending sometimes lets you download a DRM-protected PDF for a 14-day loan, which is a download in function but not in permanent ownership. It feels similar to a purchase, but the file expires.