Straight up, the internet's brimming with places for this, but it really depends on what you're after. If you want legit, high-quality stuff without any account nonsense, Project Gutenberg is the classic. It's all public domain, so you get the real deal formatted properly, no weird OCR errors. I downloaded the entire 'Sherlock Holmes' collection last week for a trip.
Some people point to sites like ManyBooks or Standard Ebooks, which are basically slicker interfaces for Gutenberg's catalog. They're fine, but honestly, I find them a bit redundant. The magic trick most folks overlook is your local library's digital portal, like OverDrive's Libby. Okay, you need a library card number, but that's it—no creating a personal account with them. You 'borrow' the EPUB or PDF, it downloads to your device, and you're reading offline. It feels almost like it's free with no signup, because you're not handing over your email or making a password for some new service.
One caveat: a lot of those 'free ebook download' blogs are just link farms pushing sketchy PDFs. I'd steer clear unless you have a good ad-blocker and a strong antivirus. The legit, no-strings-attached sources are simpler than you'd think, but they're not always the first Google result.