Apple has posted the official iPhone 4 iOS 4 user manual/guide to walk you through every aspect of iOS 4 in a 28 chapter, 244 page PDF. While this obviously focuses on iPhone 4, there are also tips for 3GS and 3G owners running iOS 4.
The guide covers 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.
You can get manuals for iPod touch and other Apple products from their Apple Manuals page.
While the majority of iPhone users will already be quite familiar with the majority of material covered in the guide, new iPhone users will definitely learn a thing or two.