Book Recommendations to Master CSS
I find CSS confusing and rely on frameworks like Bulma and Bootstrap. I want to get a deeper understanding. Are there any good books out there?
The only thing you may master by reading books is reading books. You will master CSS, if such a thing is even possible, by _doing_ CSS. Lots of it and over a long period of time.
If you are struggling with the basics, this hands-on tutorial on MDN seems to be a good start: https://developer.mozilla.org/en-US/docs/Learn/CSS. From there, it will just be practice, practice, practice…
I would suggest first identifying what you find confusing and starting there. What are the features that keep fooling you?
The box model? The cascade? Flex axes? Specificity? As a declarative language that is really intuitive to start, so simple to get so far, and one which must maintain perfect backwards compatibility with twenty years of web design, so many elements of fundamental functionality are never really studied by casual practitioners.