Well, a bit hard to say since I don't know how much you know now and how much you actually want to know. There's lots to read on the intergeek.
A good place to start, actually, is
Wikipedia for a quick overview. There are articles on various topics related to all things networking, and links to external resources.
A Linux-specific
network admin guide is freely available on the net. Yes, it's about Linux but the introductory chapters are about networking as such and the principles are the same, regardless of the OS you choose to run.
Linux Networking-concepts HOWTO also gives a reasonably good overview. Yes, it's from the freely available Linux documentation project, but that's why I like Linux, among other things: the information is freely available, and, for the most part, very accurate. You can't find many freely available MS-specific networking documents out there, as far as I know.
But also try
About.com. Lots of good stuff there, it seems, even though I tend to go for the Linux Network Admin Guide instead.