Reddit reviews Introduction to Cryptography with Coding Theory (2nd Edition)

An accessible undergraduate textbook that I have used and enjoy is Introduction to Cryptography with Coding Theory. That being said, I have not looked at many others.

A much more technical (but formally correct) textbook is Introduction to Modern Cryptography by Katz and Lindell.

On a side note, cryptography is a very mathematical field. So take as many math courses as you can. Number theory and algebra in particular.

I'm not sure what was on your midterm, if you get that back, you might want to check which concepts you lost points on (feel free to PM me if you want).

Going forward, I highly recommend supplementing your current studies by reading the corresponding sections in the Trappe & Washington Intro to Crypto book. All of my crypto classes have used it and it explains the concepts really well, as well as giving good practice problems. Thanks to those classes, I got hooked on crypto as well.

I'm currently undertaking cryptography as a unit and I'm using this book:
http://www.amazon.com/Introduction-Cryptography-Coding-Theory-2nd/dp/0131862391
It starts at the start of encrypting with caesar ciphers and goes through DES and AES in a considerable amount of depth.
By all means it isn't for the faint hearted, but you're interested enough the book will help a lot.

Without knowing how much of introduction you need, I would probably suggest Code Book by Simon Singh. It's really accessible and goes over a lot of the basic concepts (and a little history) of cryptography.

I only took one class on the subject and we used this book. I found it pretty interesting and accessible enough for self study, but I'm a casual student of the subject and really can't vouch for it's rigor or whatever.

Officially, this was the book we used for my crypto class. However, we normally just used my professor's lecture notes (I don't have them anymore, or I would post them).