Are books on iPhone books free?

Excuse me, I'm a bit confused about something I read regarding books on iPhones. I came across a statement that asked if books on iPhone books are free, but it didn't really provide a clear answer. Could you please clarify for me? Are books available for free on iPhones through some kind of service or application? Or do users typically have to pay for the books they download? I'd really appreciate it if you could shed some light on this matter.