London-based Matt Galloway, a regular presence on Stack Overflow, has published Effective Objective-C 2.0. A collection of 52 "specific ways" to improve your coding for Mac and iOS, this book provides a valuable collection of handy state-of-the-art tips that will appeal to all developers both established and new to the field.
It's a concentrated collection of wisdom with immediate and practical payoffs. From blocks to memory management (yes, even in the ARC age, you still need to know this stuff) to literals, the book is packed with helpful explanations, suggestions and directions.
It is not a long book. Although I reviewed the e-book, I estimate there's about 180 pages of actual content, but what content is there is extremely rich. I found easy-to-follow explanations and tight sample code. My favorite bits were the "Things to Remember" bullet points that peppered the end of many sections, with their must-know tips.
The book sells for US$18 for Kindle and $32 in paper format over at Amazon. Although some will read it cover to cover, I found myself picking it up, reading a section and putting it down better informed than I had started. It's a great book to sit next to you at your desk, to read a bit from when taking a break from coding.