I often ask other people what books they are currently reading. I thought you might benefit from knowing what I have been reading in 2011. Below are some selected books--in no particular order--I am currently reading/ studying.
Cocoa Design Patterns
By Eric Buck and Donald Yacktman
This book is a must have for all OS X and iOS programmers. The authors do a terrific job describing software design patterns in the context of Cocoa. The book is approachable, understandable, and down right fun to read. You will probably end up reading the book cover to cover.
https://www.amazon.com/Cocoa-Design-Patterns-Erik-Buck/dp/0321535022/
Cocoa Programming Developer's Handbook
By David Chisnall
Another great book filled with tons of great Cocoa programming topics. The book is definitely geared towards OS X. However, iOS developers will find something useful in each chapter. The appendix is one of the best gems in the book.
https://www.amazon.com/Cocoa-Programming-Developers-Handbook-Chisnall/dp/0321639634/
TCP/IP Sockets in C
By Michael Donahoo and Kenneth Calvert
Most apps do not require writing low-level socket code. Instead developers use higher level APIs that make it easier to establish socket connections for sending and receiving data. However, it is useful to have an understanding of what lies beneath the water. So hold your breath. Shove your head "under the water" (for at least a few hours). And emerge more comfortable with the C sockets API. Keep this book within arms reach. You never know when you will need it.
https://www.amazon.com/TCP-Sockets-Bundle-Practical-Programmers/dp/1558608265/
Programming with Quartz: 2D and PDF Graphics in Mac OS X
By David Gelphman and Bunny Laden
The authors do a great job describing how to use the Core Graphics API. Though this book is "old" (2005), the concepts and APIs are still very relevant to today's OS X and iOS apps.
https://www.amazon.com/Programming-Quartz-Graphics-Kaufmann-Computer/dp/0123694736/
A Book on C
Al Kelley and Ira Pohl
A great book on C. The book is a great reference, too.
https://www.amazon.com/Book-Programming-4th-Al-Kelley/dp/0201183994/
Programming Pearls
By Jon Bentley
A great collection of thought provoking questions--with selected answers.
https://www.amazon.com/Programming-Pearls-2nd-Jon-Bentley/dp/0201657880/