Very wide ranging and detailed - this book covers the whole gamut from how our eyes and visual systems and brains process images, right through to the implications for how we can visualise information in computer systems most effectively.
As a non-expert in this field I found this a dense but really rewarding read - especially as it kept surprising me with new ideas and information about how we see the world! There were too many "wow, really?" moments for me to easily remember.
For instance, I had no idea that as well as colour blind people, there are a very small number of people who have four distinct colour cones in their eyes, allowing them to distinguish more colours than the vast majority who have three.
Just a note for readers - this is a foundational text - it lays a groundwork of information, I found it fascinating but it's less of a how-to guide than a "why you should approach visualisation in these ways" guide. I'm giving it five stars for how well it accomplishes it's aims - but it might not be the book for you if you just want to jump in and build data visualisations, or if you want a light easy read!