A very brief intro with a few examples to areas such as prototyping, users psychology, usability, and animations, each of which is probably better introduced with a separate book. And the general lessons (especially the perspectives in the interviews) would be better understood if combined with some hands-on experience. So I would say it's a light reading for intermediate UI people. (Beginners like me might want to look for something more concrete before returning to this).
A partial summary of useful principles:
1. Know the users well. Build a product that actually cures users's pain instead of perfecting a product that serves an empty need.
2. Avoid the pitfall of user behaviour research: asking people to verbally tell what they (think they) need (or, say, how much they like a product), instead of observing what they genuinely need (or how frequent they use a product).
3. Prioritise different stages of design. Start with descriptions of the essential contents or personality, and then the basic skeleton, before diving into subsidiary elements like fonts or colour.
4. Minimise awkward UI situations, like blank pages (use loading indicator, or partially load the skeleton of a website), extended loading time (which might dramatically reduce the number of times a user uses a function)...
And much more besides these.