Chess books for difficult endgame studies

I read a chess book awhile ago that was a collection of really hard endgame studies. Each one was almost impossible at first and when I saw the answers they had a really deep, instructive solution. I believe there was a chapter purely for (extra) difficult studies. Does anyone know of a book like this? I forgot what it was.

sorry, does not sound familiar.

Many books might fit your description, but one that I know of is AJ Roycroft's The Chess Endgame Study.  It's a Dover book that was originally published in the early 70s under the title Test Tube Chess.

Maybe the book you are looking for is referred to in one of these threads:

https://www.chess.com/forum/view/chess-equipment/good-books-for-improving-your-endgame

https://www.chess.com/forum/view/chess-equipment/best-composition-collections-endgame-studies-and-problems

I think this sounds like one of Troitzky's books. I forgot the title but it's something like " Troitzky 300 endgames composition"or something similar.

Domination in 2545 endgame studies by kasparyan 

Domination in 2,545 Endgame Studies https://g.co/kgs/SbNjY9