A couple of months ago I started working on a Typo3 project for a client, and at about the same time I was asked by the folks at Packt Publishing to review a book, so the choice fell naturally on "Typo3 Extension Development" by Dmitry Dulepov. The book, published in September 2008, covers the entire extension development life cycle, from planning to uploading it to the Typo3 Extension Repository (TER).
Starting with the table of contents, he goes through the book and shares opinions about each step of the way. He liked the book for its simplicity and straight forward content and loves the "immediate head-start" it gives to the reader. The only thing lacking was the term index that didn't seem to add much to the content due to its small size.