I separate my learning into “understanding/” and “skill”, the latter being the applied form of the former. I don’t really buy books for the latter, and the former may not be always in book form.

IE6 hacks are skills. The real understanding is how CSS helps with layout. Grid may not be in an old book, but the old will give you enough foundation to easily grasp it. So something like “CSS in Depth”[0] (which talks about grid BTW) is a good candidate to buy.

I think you can get good materials if you focus on getting books that talks about a system (design, theory, implementation), not techniques(skills, how to). Systems evolve and are build on top of each other, or influence each other. Skills come and go.

[0] https://www.manning.com/books/css-in-depth