Michael Bluestein is a software developer, consultant, and active participant in the MonoTouch developer community. A former Principal Software Engineer at Dassault Systèmes Solidworks Corporation, he has developed software professionally since the early 1990s. Michael is the author of Learning MonoTouch: A Hands-On Guide to Building iOS Applications with C# and .NET.
Cocoa Design Patterns, by Erik M. Buck and Donald A. Yacktman
This is a fantastic book for understanding the concepts behind the design of Cocoa and how to apply them in your own applications. Also available in Safari Books Online.
Cocoa® Programming for Mac® OS X, 4th Edition, by Aaron Hillegass
The third editon of this book is the first book for developing on Apple technologies that I read. If you have development experience on another platform and want to get started with Cocoa, this is where to begin. [Editor's note: We've linked to the 4th edition above, which will be available in early- to mid-November 2011. Until then, the 3rd Edition is still available.] The third edition is also currently available in Safari Books Online (4th Edition coming to Safari Books Online soon).
Expert F# 2.0 (2010 Apress, ISBN 9781430224310), by Don Syme, Adam Granicz, and Antonio Cisternino
Learning F# is one of those things that will make you a better programmer. Lots of interesting ideas from F# are making their way into C# as well.
C# in Depth, Second Edition (2010, Manning Publications, ISBN 9781935182474), by Jon Skeet
I've written a lot of C# code and read many C# books, and this one is the best hands down. In fact, it's not even close. There is a new edition out now too. Just buy it.