How do I get books onto my iPhone?

Hello, I'm curious about how to get books onto my iPhone. I've heard that there are a few different ways to do this, but I'm not sure which one is the best or easiest. Could you please explain the different methods to me and help me decide which one to use? Also, are there any specific apps or services that you recommend for downloading and managing books on my iPhone? I'd appreciate any guidance you can provide on this topic. Thank you!