fortress positions

Keeping hearing about these. I understand the principle but is there somewhere they are set out in a book? How useful is knowing them?

The wiki article (http://en.wikipedia.org/wiki/Fortress_%28chess%29) lists quite a few, though their definition is a lot broader than mine (for example I wouldn't consider KB + rook pawn of the wrong colour vs K a fortress position).

Personally I've never found knowing about fortress positions to be particularly useful. If you find yourself wanting to study endgames, then learn as much as you can about king and pawn endings. They crop up all the time!