I don’t know if you have heard about Wikibooks yet or not. If you haven’t then you are missing out on a great resource. Basically Wikibooks is a community for creating a free library of educational textbooks that anyone can edit. Sort of like Wikipedia but specifically for Books.
One of the books they have is a book on Cryptography that is quite easy to read and follow. At the time of this writing a lot of the sections in the book still have to be added but new content is added regularly and over time I think it will become a great resource for everyone.
Check it out.
– Suramya