when they say it's a matter of technique it usually means converting a theoretically winning endgame, so yeah... endgame books.
Chess Technique and Books
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Soltis' "Turning Advantage into Victory in Chess" gets a huge recommendation from me.
It's not just an endgame book (it has some transitional phases), and even has a part where the winning side trades the more active rook because, "it isn't what leaves that matters, but what stays on" If you're winning far more clearly without the rooks and their king on the recapturing square, then by all means fall for their trade offer! Don't just keep it because some book states a general rule to keep a more active rook (which is generally good advice, don't want to trade down from a winning to a drawn position).
TL;DR: Soltis' book on technique is great, and if you evaluate a simplified position as winning then don't be afraid to cash in your advantages to convert the win.
You always read or hear the phrase, "the rest is a matter of Technique." What does that mean? Is there an exact definition of what Chess Technique is? Also, are there any good books that are still in print which teach Chess Technique?