Chess Endgame Lessons is one of my favorite endgame books. This is a collection of 10 years worth of Benko's first-rate endgame columns in Chess Life, along with 30 of his games which moved into the endgame stage, with detailed annotations. I grew up with these columns, which address nearly every conceivable aspect of endgame theory, always thematically unified so that even inexperienced players can learn the basics of each type of ending. But there's plenty for the advanced player as well. Benko is also a brilliant composer, and I suspect that he ranks very highly among all living endgame connoisseurs. I strongly recommend this book (if you can find it--it's self-published). I have also heard that there is a more recent collection of his later columns; I haven't seen it, but I'm sure that one could never go wrong with this brilliant teacher.