eBooks
There are rather small companies who publish ebooks or books on demand but nonetheless their content is great! Plus PDFs are searchable.
I found some great books at pragmatic programmers, all of them are really pragmatic and more than worth their money: Easy to read, no fuss, just information you need:
Andy Hunt, Dave Thomas
Pragmatic Unit Testing
in Java with JUnit
ISBN: 0-9745140-1-2
Great introduction in the various ways to implement unit tests
Venkat Subramaniam, Andy Hunt
Practices of an Agile Developer
Working in the Real World
ISBN: 0-9745140-8-X
One of my favorites: be agile, avoid mistakes so many developers (and managers) made - embrace change.
Andrew Hunt, David Thomas
The Pragmatic Programmer: From Journeyman to Master
ISBN: 020161622X
This was their first book and still has many valid tips on pragmatic programming.
Printed Books
Aaron Hillegass:
Cocoa Programming for Mac OS X
Addison-Wesley, Pearson Education, 2002
ISBN 0-201-72683-1
(second edition 2004: 0-321-21314-9)
This would be the traditional starter book for Cocoa programmers. All examples are easy to follow and show basic needs. It's fun to read and useful even afterwards.
If you already know C this is a really good start!
Scott Anguish, Erik M. Buck, Donald A. Yacktman:
Cocoa Programming
SAMS, 2003
ISBN 0-672-32230-7
This is a reference book with 1150 pages full of detailed information. I oftentimes found the right clue which wasn't in the documentation. They are experienced authors who introduce the reader to the large framework and follow up with programming techniques and the Cocoa runtime.
Even though it is dated by now (was released when Mac OS X 10.2 was current - IIRC) it has still a lot of useful information.
Joe Zobkiw:
Mac OS X
Advanced Development Techniques
SAMS, Developer's Library, 2003
ISBN 0-672-32526-8
Zobkiw takes the "learning by doing" approach providing a solution to common problems like programming plugIns, creating frameworks, SystemServices, PreferencePanes and StatusItems, coding a ScreenSaver etc. He also has included interesting chapters like Threads, XML-RPC and SOAP under Cocoa.
This is an ideal addition to the books mentioned before. It's easy to read and expands your knowledge. Really recommended!
Mark Dalrymple, Aaron Hillegass:
Core Mac OS X and UNIX Programming
Master Darwin and the Core Technologies
Big Nerd Ranch, 2003
ISBN 0-9740785-0-6
This definitely is a book for nerds - not really suited for beginners. After a short intro to C the authors take you on a tour through libraries, memory, exceptions, debugging, I/O, sockets, multi-processing, Apple's keychain, RendezVous (Bonjour), threads, CVS and performance tuning.
Not necessarily something you will read before you go to bed but something to have at hand when the going gets rough. If you're a pro it's a must have!