If you're a Mac or iOS developer and happen to have an iPhone, iPod Touch or iPad running the iBooks app, go open the iBookstore and search for "apple developer". As you can see, Apple is offering iOS / Mac development iBooks completely for free.
The six books, published by Apple Developer Publications, include "iOS Technology Overview", "Cocoa Fundamentals Guide" and the popular "iOS Human Interface Guidelines". Some books report a release date of "November 2010", but Apple is making sure you're running the latest iBooks version by writing in each description "This book displays best with iBooks 1.2 or later".
Indeed the books are elegant and come with a lot of detailed graphics and screenshots. Sure they're not illustrated books (supported in iBooks 1.2), but I can see why Apple is recommending the latest version of their ebook reading software.