Also zeal (dash on Mac which is paid or you can build zeal yourself like I did) has docsets for CSS which are good.
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.
I’m currently writing my second one, which will be more theoretical and cover things like this blog post actually. I’d be interested to know what kind of problems you’re facing in CSS, so feel free to drop me an email.
Sorry for coming out of nowhere, but I have lots of small tutorials I could use (I teach in a local college) which I could make small ebooks out of them.. Looking for some side money..
As for technologies, I just wrote the book in markdown using Sublime Text. Then I used a markdown-to-pdf library from GitHub to generate my ebook, and voilà. Nothing fancy really.