It delves deep into the underlying concepts for CSS layout as described by the spec. I think layout is the hardest part of CSS, as changes can potentially affect the whole page, so it's worth studying the fundamentals.
It was written before grid was a thing, but it does cover flexbox.
I do wish CSS layout was easier to understand. Perhaps CSS grid will change this as it becomes widely adopted.
http://book.mixu.net/css/
It delves deep into the underlying concepts for CSS layout as described by the spec. I think layout is the hardest part of CSS, as changes can potentially affect the whole page, so it's worth studying the fundamentals.
It was written before grid was a thing, but it does cover flexbox.
I do wish CSS layout was easier to understand. Perhaps CSS grid will change this as it becomes widely adopted.