It is a classic of its publisher and it moves in parts: composition and the grid, colour and its theory, texture, typography and imagery. Each chapter takes real sites apart to show why they work, and that exercise is what justifies reading it.
What I took from it
The vocabulary. Being able to say that a page fails for lack of hierarchical contrast, or that the problem is vertical rhythm rather than colour, changes a conversation that would otherwise stall at something looks off. With clients and with developers, having the exact name for what is wrong saves two meetings.
Colour theory applied with a simple rule: one family of tones, one accent, nothing else. And the explanation of why certain combinations tire the eye, which is a better argument than personal taste when a decision has to be defended.
And a compositional idea that remains true: empty space is not wasted space, it is what makes the thing beside it readable. The chapter that takes finished sites apart is worth more than the theory around it, because it shows the reasoning rather than the rule.
What I left out
A good deal, for reasons of date. This edition is from 2014: it writes when designing for phones was still a separate chapter rather than the starting point, system typefaces and relative sizing were at another stage, and several of the sites it dissects no longer exist. For practical decisions today I prefer a later, shorter book; I keep this one for the composition chapter, which depends least on the technology of its moment.