If you're looking to find out what your iPhone is capable of and pushing it to its limit, then this is for you. Stefan Neagu's The Unofficial Guide to the iPhone is a free PDF download that you would want to consult.Link: The Unofficial Guide to the iPhone
"Read about the basic user interface and a ton of incredible iPhone features you would’ve otherwise missed. Stefan explains in detail how to perform both the very simple and the most tedious tasks. Find out how to get your hands on fresh applications, how to keep your device synchronized and even how to jailbreak your iPhone!"