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Re:Things I wish I’d known about CSS by noir_lord on Friday July 17, 2020 @05:59AM (#23868948)
Not a book but https://developer.mozilla.org/en-US/docs/Web/CSS is my go-to for all things CSS, I love mdn it's a fabulous resource.

Also zeal (dash on Mac which is paid or you can build zeal yourself like I did) has docsets for CSS which are good.

Re:Things I wish I’d known about CSS by dwd on Friday July 17, 2020 @06:23AM (#23869068)
https://caniuse.com/ is also invaluable to check whether the cool CSS feature is actually supported. it's very much today's quirksmode.com.

Once upon a time I would have recommended Eric Meyer's CSS Definitive Guide without hesitation (I think I still have my 1st Edition copy) and it looks like he's re-released with updates for flexboxes and grids.