I use propokertools.com to better understand spots and review
hands. (their simulator is awesome for working out EV models)
Not sure about sites, but theres several good books.
Theory of Poker- Skelansky
Kill Phil/Kill Everyone - Nelson, Streib, and Hieston
Super System - Doyle Brunson (very dated, but gives good fundamentals)
another book i hear about regularly (which i have not read)
The Raisers Edge - "elky" grospellier