Well, there is more resources about on backgammon on net which is also free and very helpful.
Try with www.gammonline.com
and you don't regret. It is Kit Woolsey online magazine which is now open for read only. One another site www.bkgm.com
(Backgammon Galore) by Tom Keith is also excellent. And www.bgonline.org
by Stick Rice. You can find usefull free articles by Walter Trice, Mary Hickey, Paul Money and others on www.gammonlife.com
All these places with GNUbg and FIBS are must for any serious backgammon student and online player.
Btw, Robert-Jan Veldhuizen, the author of GNUbg tutorial which you mentioned also play on FIBS.