I would read Beyond the Boxscore and the posts on Fangraphs.

another good place to start would be Baseball Between the Numbers, by the BP guys, which is a good read and introduces a lot of the basic sabermetic tenants . its also cheap as hell on amazon used:

http://www.amazon.com/Baseball-Between-Numbers-Everything-About/dp/0465005969

The stats glossary mini-series that some of the guys here did was also cool.

http://www.royalsreview.com/tags/stats%20glossary?order=date&type=FanPost

Overall though, I’m not sure there’s a definitive “sabermetrics” per se at the moment, beyond the stuff that the pros are doing, most of which is being done for teams and isn’t available publicly. I’d say, for many of us, it’s more a mood, or an attitude, along with a handful of basic ideas. Past that, and it can be a lot of things to different people.

but now that it’s a pay site, not sure it’s for everyone

The HardBall times is also good

they don’t go into tremendous detail, but they give some explanations of what their doing in their glossary and also some excerpts from The Book

http://www.statcorner.com/index.php

www.fangraphs.com

www.baseballprospectus.com

http://www.insidethebook.com/

and as RR suggested, THT.

I would also recommend:

Statistically Speaking
http://statspeak.net/

By the Numbers (free to download)
http://philbirnbaum.com/

Baseball Between the Numbers is by far the best primer out there.

some of us started a series of posts here at Royals Review called Stats Glossary that contain some basic concepts explained using Royals players.

As part of that series, i put together a post with some links to specific postings I found helpful: Attention Aspiring Nerdlings.

Another specific group of posts at the aforementioned Beyond the Box Score is Sky Kalkman’s Saber-Friendly Blogging 101, which is mostly did last season, but which I have on good authority is going to be picked up again, soon. It does a good job of explaining not just what some of the more contemporary sabermetric concepts are, but easy ways to calculate them yourself and explain them to others.

If you aren’t overloaded yet, another good reference resource that Tango’s Sabermetric wiki.

Baseball Between the Numbers, as ofthers have said, is excellent. Once you’ve read through what you want of that, I strongly recommend The Book, which is not only more sophistaced, but, while a bit dry, is well-written and packed with information on strategic questions previous sabermetricians haven’t addressed that well for lack of data. It’s worth noting that all three authors have actually worked for baseball teams. Plus, a big portion of the publicly-available sabermetric work being done is indebted to these guys.

Don’t be intimidated: Both it and BBTN are the kinds of books where you can read the intros then skip around to read the chapters that interest you at the moment.

Above all, don’t be afriad to admit when you aren’t sure about something or need something to be explained. That’s how I continued to learn. If someone is going to be an arrogant jerk and look down on you when you’ve asked politely, then their opinion likely isn’t worth hearing. In my experience, pretty much everyone online is more than willing to try to explain what they’re saying in terms I can understand. And believe me, I need stuff laid out really simply.

Reading Bill James, Rob Neyer, and hanging out at Baseball Think Factory.

I still learn from reading this site, Hardball Times, and Fangraphs