I'm guessing he has a general idea of what he is looking for, but won't know more until he digs into it. Oddly enough, I'd think the internet rather than a book is the way to go. Hyperlinks will help him explore the ideas. I'd start him with Wikipedia, specifically on Internet Protocol: http://en.wikipedia.org/wiki/Internet_protocol

That can lead to messaging schemes, security, encryption, scripting, markup, etc.