Instruction manuals for the Apple iPhone 3G are available at Apple.com. To access the manuals go to the Support section of the website and select iPhone. Type "iPhone 3G" into the search bar to see only iPhone 3G manuals.
Apple's online manuals for the iPhone 3G include the Product Info and Safety Guide, Finger Tips Guide and iOS 3.1 User Guide.
Printed versions of Apple's iPhone 3G manuals are available for sale on websites such as eBay. Many sellers include the original manuals as part of a set together with the original boxes for collectors and resellers.
Unofficial manuals, such as David Pogue's "iPhone: The Missing Manual," cover topics including iOS 3.1 and iPhone 3G hardware functions.