Apple has produced six free developer guides as iBooks that can be read on an iPad or iPhone. They are available without charge from the Apple iBookstore and work best with the recently-released iBooks 1.2.

The six free  titles are: 

• iOS Human Interface Guidelines
• Object Oriented Programming With Objective-C
• Cocoa Fundamentals Guide
• The Objective-C Programming Language
• iOS Technology Overview 
• iOS Application Programming Guide

The publications can also be downloaded directly from the Apple Developer's Center (no registration required)

http://developer.apple.com/devcenter/ios/index.action

where they can by read online and downloaded in PDF format.