David Pogue's popular help book — iPhone: The Missing Manual — is now available as an iPhone application. The book costs $24.99, but the app is available at a special introductory price of just 4.99. (Regular price is 9.99.) This second edition has new material related to the iPhone 3G and MobileMe. Plus, there are new chapters on the app store and Microsoft Exchange.
Pogue's books are very popular with computer users, because they generally do an excellent job of not only teaching you all the features but also offering tips and tricks not usually covered in the manual.
If you don't want to spend the money, you can also download a free PDF manual from Apple.