William Stallings Hardcover, 2nd ed. It provides an excellent introduction to these concepts as well as a useful reference. A gift from Cisco I received this book from Cisco as a gift for filling out a survey they did. I knew I had struck gold. It's a rich text that really gets you interested, and is remarkably clear about some difficult subjects.
Stallings is a wonderful author.
We learn about DES in detail because it provides a fantastic platform to understand modern Feistel based methods, which underlie everything from DES to Idea and other modern private key methods. We examine a few rounds and then get to perform multiple round DES by hand. Tedious, but you really learn how transformations work. Through it all, Stallings does a fantastic job of teaching us the basics. His notation is really what makes the book excel.