Honestly, the obsession with 'free' makes me a little tired sometimes—you get what you pay for, and that often means a mess of pop-ups or machine-translated nonsense. Legit free options exist if you're patient and know where to look. Project Gutenberg is the classic for public domain stuff; tons of older foundational fantasy is there, like William Morris or Lord Dunsany. It's not shiny, but it's solid. For newer works, many authors put the first book of a series free on Amazon Kindle to hook you, which you can read in a browser via the Cloud Reader. That's a strategic free, not a sketchy one.
My dark horse recommendation is actually library digital services like Libby or Hoopla. You need a library card, obviously, but that's free too. The selection varies wildly by location, but when it's good, it's very good—full, legal copies of major recent releases. Waitlists can be long for popular stuff, but it teaches delayed gratification, which feels appropriately fantasy-quest-adjacent.