Josuttis’s books are normally pretty good, lots of examples and a clear explanation of why you might want to use something, but oof that looks akin to a kick in the essentials.

Even if you’ve no other reason to update to C++20, the fact that if constexpr gets rid of half the things you’d previously need to use SFINAE for, and concepts gets rid of the other half, makes it well worthwhile. Amazing how much it stops hurting when you stop doing ridiculous things.