If you are looking for previously unpublished analysis and explanation of practical endgame positions from grandmaster play, the book I am going to review is the right fit. You would be able to use it for learning advanced endgame concepts, as well as practicing your skills via exercises.
I read Endgame Secrets by Christopher Lutz a long time ago, but picked it up from the library yet again last week. Often endgame books dive into tons of theoretical positions, which are either too simple, or too unlikely to occur in any one of your games. This book is a collection of analysis from practical games (majority played by the author himself), so you’d be asked to evaluate the position, or come up with a plan.
The author gathered material for the book as he realized that he was losing a lot of points in endgames, and decided to improve his own endgame skills. Games are sorted by chess material (e.g. rooks + bishops of opposite colours), that helps to see the patterns as well. If you are rated above 2000 – I recommend studying this book to you, as well as analyzing your own games the way Lutz did.