http://gosu02.tripod.com/id13.html http://longgandhi.com/formulas.html are some starts. I think ERC is in the back of some BJ books, but not handy.
DIPS has a ton of articles, easy to find froms searches here. I think Jim Furtado's XR is available from either BigBadBaseball.com or from a link on baseballthinkfactory.org (under stuff).
To read about WS, go to Stude's website
www.baseballgraphs.com -- he also has a link to a WS spread sheet. Sometimes it is easiest to see things from the number crunching.
Anyway, those are starts.
edit: czzccr beat me to 1/2 of it! thanks!