Apple has released the iPhone user guide for iOS 4 software. It's a 17MB, 244 page PDF designed to walk you through everything from syncing to using the default apps. The 28 chapters cover the Phone, Mail, Safari, iPod, Messages (SMS/MMS), Calendar, Photos, Camera, YouTube, Stocks, Maps, Weather, Voice Memos, Notes, Clock, Calculator, Settings, the iTunes Store, Compass, Contacts, Nike + iPod, iBooks, and Accessibility ... whew.
The book includes tips specific to the iPhone 4, but it also includes tips for the 3GS and 3G. When a section of the manual is only applicable to a certain device, it is noted at the beginning of the section.
As usual, the iPhone iOS 4 user guide is well-done. If you've been using an iPhone for any length of time, you've probably figured out most of this stuff already. It's a good resource for new users, though. There is a whole section of Apple manuals for various devices including the iPod and iPad.