I’m going to make a couple of recommendations, but I do have a question - why are you looking for free/out of copyright books? These have a couple of issues that may get in the way of your primary goal of getting g better at reading and, I assume, learning about new subjects. I’m also going to make the assumption that we’re talking primarily about English language books, but note that you didn’t specify a set of topics.
Many of the books that you can find on, eg, Gutenberg, suffer from being poor or outdated translations. If you’re really looking to understand Marx’s Capital (to take an extreme example) I could not recommend a resource less than Gutenberg. It is atrocious. If you want to read Dickens or something, it is at best plain unflavored oatmeal. I’d like to suggest a couple of alternatives.
When I had zero income for a while and was simply burning through my savings to live from day to day but still needed to read and learn - both to feel human and to move on to my next phase in life - I found torrents of ebooks. Some of them were just crappy PDF scans where the pages were just images (I think my first Zizek book was like that), while others were available in or translatable to an e-book format. The ones I tended to grab (and this is 20+ years ago) were things like the entire collection of Oxford University Press books for a span of years, which would cover science, philosophy, literature, and so on. Each one was gigs in size, and I used an ebook program to catalog them.
The other option, depending on where you are or where you can manage to appear to be, is the public library system. This lets you borrow many books, and libraries aren’t all that strict (generally speaking) about validating addresses and such.
My suggestion overall is to read about reading, and in general to read more modern books. If you’re interested in Jack London, don’t just read Jack London. Read about how he fits into English literature by people who have dedicated their lives to the subject. Read about the world he inhabited, what his metaphors mean, and how he compares to writers in his genre who preceded and followed him.
Oscar Wilde is a darling author, but again you need the full literary context to really appreciate gay history and literature, you’ll want the additional history and context of his contemporaries, his historical conditions, and how his work influenced future authors.
While it’s easier to appreciate Sherlock Holmes than Shakespeare, you won’t get as much out of it out of either without a bit more digging.