[mattbee]

http://www.htmlandcssbook.com/ taught me the groundwork despite its age. Lea Verou's CSS Secrets is a great book to pair it with - will show you lots of avenues of CSS and layout ideas that you'd never have found on your own. You probably also need some reference on Flexbox & Grid but depends how far you want to go!

[teniutza]

CSS Secrets is wonderful but I wouldn't consider recommending it for learning CSS as it assumes at least a solid base. What I do believe someone new to CSS or currently learning would find useful is the overall approach. Until reading it, I did not really take any interest in being DRY with my CSS.