A lot of comments here already address how to get into the learning side, and the theoretical side. If you also want to get your hand dirty and learn some more

A lot of comments here already address how to get into the learning side, and the theoretical side. If you also want to get your hand dirty and learn some more that way (eg: experience), get some VPS with BSD (my preference is OpenBSD) or some really bare linux distro and set up your own email server (eg: OpenSMTPd), IM server, etc there. You'll learn plenty on the way: about servers themselves, how emailing works, management, etc, etc. I learnt huge amounts doing this years ago, and have an extremely detailed understanding of how email, anti-spam, validation, etc work. Don't do this "instead of" grabbing books though, do it "on top of", especially if you want to make a career out of it in future.