Hi Nit,

I'm not sure what level you are looking at but Simon Singh's 'The Code Book' makes a good read and gives an historical account of the development and use of codes and cracking, including Caesar shift, a polyalphabetic caesar(the name of which I can't remember), Playfair, ADVGFX (or other letters - I've forgotten), Enigma, and a brief introduction to RSA.

http://groups.yahoo.com/group/CipherChallenge

is a forum that has been running for quite a while to solve the £10,000 challenge at the back of the book - sadly this was won last year so the incitement to solve the puzzles has somewhat diminished, but there is a wealth of information there. The forum is very much related to the book although since the challenge was completed other subjects have crept in.
I think you have to register and then join the group- just a few clicks and a made-up name :7)