A few days after Jobs, Dennis Ritchie and John McCarthy passed away too, but that did not trigger international interest.
Dennis Ritchie is called the "father of C", C as in "C language". Everyone who's done a little bit of programming knows about the importance of C in the computing world.
A few days after Ritchie, John McCarthy, the "father of Lisp" passed away too. Lisp is the language that was mostly used for artificial intelligence works "back then".
Lisp is 11 years older than C. Lisp was born in 1958 and C in 1969. But both languages are still commonly used in computing today...
Of course, both languages can be used on Macs. If you install the developer's tools that come with your DVD, you have access to a C compiler. Lisp, being a family of languages, requires to make a few choices (either get an ANSI standardized Lisp, or a Scheme, or a new Lisp like Clojure, that runs in Java etc.)
As for introductory books, "Land of Lisp" by Conrad Barski, M.D., from No Starch Press has been very well reviewed.
"Practical COmmon Lisp" by Peter Seibel from Apress is really nice too and sparked a renewed interest in the language. Plus, the PDF is freely available.
As for C, well, there are so many books about C programming that the only one I can think of is Kerninghan and Ritchie's "The C Programming Language, Second Edition" from Prentice Hall.
Objective-C is a strict super-set of C and is mostly known for being the language behind OSX applications.
A good introduction I found is "Programming in Objective-C" by Stephen G Kochan, from Pearson Education.
Of course, you can find plenty of free tutorial that can get you started in both languages.
Programming is fun and if it is not already the case, you should really give it a try.