Personally, I would seriously recommend to read 501 BG Problems of Bill Robertie. If you are looking for an Internet site, try
Backgammon galore. They have a lot of good articles there and it is free.
There's one absolutely fantastic
article there, in my holy opinion, that really opened my eyes. It is an article about a short match between Kit Woolsey and Jeremy Bagai, which was annotated afterwards by the players and supplemented by the evaluations of TD Gammon, the first neural network backgammon engine (thus an ancestor of Snowie and Co).