Where can I buy books on my iPhone?

Excuse me, I'm in need of some guidance here. I've been wanting to purchase some books on my iPhone, but I'm not sure where to look. Could you possibly enlighten me on the best places to find and buy books directly on my device? Are there specific apps or online stores that cater specifically to this purpose? I'm quite interested in both fiction and non-fiction titles, so any suggestions would be greatly appreciated. Thank you in advance for your help!