How to buy books for iPhone?

Excuse me, could you please clarify the process of purchasing books for an iPhone? I'm quite interested in expanding my reading library but I'm not entirely sure how to go about it. Do I need to download a specific app or can I do it directly from the device? Are there any limitations to what types of books I can buy, or is the selection vast? Also, is there a way to preview the books before making a purchase to ensure I'm getting exactly what I want? Lastly, how does the payment process work, and are there any additional fees I should be aware of? Thank you for your time and assistance.