Not a book but a service. Books24x7
If your company doesn't offer it, get them to offer it. Its what O'Reilly's Safari is trying to be but it has more publishers.
Learn Objective-C for Java Developers
by James Bucanek
Apress © 2009 (519 pages) Citation
Great book to learn objective-c if you know java.
4 out 5 five stars (not a great reference after you've read what you need).