[dfan]

This is different from the other answers, but it does answer your question: When I was a kid I had tons of math and logic puzzle books. Two I remember specifically are "Aha! Insight" and "Aha! Gotcha" by Martin Gardner. Decades later, when a math problem comes up in my work, I have an apparently unusual ability to cut to the heart of it ("by symmetry, we must have X" or "looking at this extreme case, we must have Y" or "this looks like a special case of Z" sort of things) instead of starting by soldiering through equations, and I credit a lot of that to all the puzzle-solving I did as a kid.

[wannabebarista]

I had a similar experience with Raymond Smullyan's books, particularly The Gödelian Puzzle Book: https://www.raymondsmullyan.com/books/the-godelian-puzzle-bo.... Recreational math is quite underrated.

[macrolocal]

A Mathematical Mosaic is a little-known gem here.