The first book I bought to try to get new ideas on how I could better leverage my iPhone as a multimedia Swiss Army Knife was iPhone Fully Loaded by Andy Ihnatko. I have been really impressed by this book because it has some really excellent tips and techniques that go beyond many of the ideas I've seen discussed on iPhone-related blogs and websites.
Andy Ihnatko is a freelance journalist who writes a technology column in The Chicago Sun Times and appears regularly on The Early Show on CBS. He hit my radar screen through his regular gigs on The Apple Phone Show and MacBreak Weekly podcasts. Some of the concepts he discussed on those programs, such as using Smart Playlists to fill your iPhone with a constant amount of music that you like but haven't listened to recently (mentioned previously on Operation Gadget), and using Handbrake to convert chapters of DVDs that you own to clips that are playable on your iPhone, are prominently featured in this book. However, there are a lot more ideas that go far deeper into Mac and PC technology to pull together content that you have access to, package it in a form that's storable on your iPhone or iPod touch, and get it transferred on to your device.
There are also ideas that didn't appeal to me personally, but were interesting to read about from a general knowledge perspective. Andy is a big fan of comic books, so he includes an entire chapter on finding comics on the Internet and transmogrifying for your iPhone. He also talks about extensively about electronics and software that can be used to record radio programs for later playback on your iPhone. I used to listen to a great deal of radio myself, so this is interesting to me, but podcasts have largely replaced my radio listening habit since I got my iPhone, and I can barely keep up with the podcasts that I'm subscribed to now.
There are a number of other good iPhone-related books, such as The iPhone Pocket Guide by Chris Breen of MacWorld Magazine, but few are as jam-packed with ideas for filling your iPhone with content as iPhone Fully Loaded. This book always seems to be sitting near my MacBook Pro, and I think it will stay there for some time.