Lured by the iPhone platform, this is what I read in the evenings. Feels kind of funny: after 10+ years of Java/C# and almost as many years of C++ beforehand, I mostly lost my C instincts and C habits. Fortunately, I have also mostly lost my C++ bad habits as well.
The Apple site is definitive starting point - put the http://developer.apple.com/referencelibrary/index.html into your bookmarks. From non-Apple sources, very bookmarking-worthy is the Cocoa Literature and Cocoa Devcentral.
Theocacao has a nice example highlighting diferences between Objective C 1.0 and 2.0. There are many tutorials available, with various depth and quality, e.g. here, and here. Wikipedia page is also quite useful.
From books - I have browsed few in Chapters and on Safari Books, the best still seems to be Hillegass's book. There is an updated version (for Objective C 2.0) coming out in June 2008.
From list of links, try this.