iPad User Guide for iOS 5.1 is available in eBook format, as a free download from the iBookstore.
Here’s everything you need to know about iPad, in a handy eBook format. Get to know iPad and discover all the amazing things it can do, and how to do them. It’s the definitive guide for getting the most from your new iPad, straight from Apple. The iPad User Guide is an essential part of any iPad library.
Also, here are the updated User Guides for iPhone and iPod touch:
iPhone User Guide for iOS 5.1 by Apple Inc.
iPod touch User Guide For iOS 5.1 by Apple Inc.