Dont know your level and haven't read Hellsten's book but Shereshevsky's is easy to work through and learn from.
Aside from those two, I can recommend Chernev's book 'Capa's greatest endgames', Giddins, Greatest ever chess end games and Marin's book -something like 'Learn from the masters'.
In your place, I'd get the one that seems at the right level and is least expensive and start with that one. Bill
I would like to improve my endgames in general, and I think I know pretty well my theoretical endgames, so I would like to improve my endgame strategy. I've done my research and found that these two books are very good. If you had to pick one, which would it be and why?