I was wondering the same thing myself. I know it's unscientific / biased, but I usually take a quick peak at the Amazon reviews. Here's the summary, in case it helps anyone else:
- Programming Voice Interfaces = no reviews
- Mobile App Development with Ionic = 3.5 stars (3 reviews)
- JSON at Work = no reviews
- Refactoring JavaScript = 5 stars (2 reviews)
- CSS Refactoring = 4 stars (4 reviews)
- Interactive Data Visualization for the Web = 5 stars (16 reviews)
- Learning React Native = 5 stars (2 reviews)
- Learning HTTP/2 = 4.5 stars (7 reviews)
- SVG Animations = 4.5 stars (8 reviews)
- Working with Static Sites = no reviews
- React Native Cookbook = no reviews
- CSS: The Definitive Guide = 5 stars (13 reviews)
- Using SVG with CSS3 and HTML5 = 5 stars (1 review)
- Learning React = 4 stars (22 reviews)
- High Performance Images = 5 stars (1 review)
The sample size of reviews for most of the books is a little limited, but I'm planning on picking up the bundle at the $15 level anyway -- CSS: The Definitive Guide is well-reviewed (and expensive by itself).
Yes, I understand it's totally unscientific to ask that question on random internet forum but I'm trying to control my I'm impulsiveness and make more rational buying decisions.
This is my first time ever buying one and I know these books individually cost more than $15. I have a dream project that involves voice, I want to know more about HTTP/2, and I could always improve my JavaScript skills.