Love your iPhone? Bet you don't use all the features and shortcuts it has ...
My iPhone 4 by Brad Miser is a great read and reference manual for the iPhone 4 (as well as 3 and 3GS). It's hard to write a computer book that meets the needs of both beginner and advanced users, but I think that this one does. Screen shots help after the description. Available in print for $16.49 and on the Kindle for $9.99
I've been using an iPhone (well, more than "an" since I keep upgrading to the newer model and passing down my older ones) for 3.5 years and I learned some great new tricks. It has both suggestions and warnings.
Some like to read a book from front to back - you can also skip topics you're not interested in and come back to read them later if you become interested in a topic.
This is good for beginners, as it explains basic words like "tap" and "drag" but does so concisely, so that it doesn't bore the advanced user who can benefit from things that aren't in the manual and aren't intuitive.
Remember - with the free Kindle app on your iPhone, you can download this book to your iPhone and have the reference always on hand.