Prose that causes you to envision beautiful landscapes, and prose that is simply beautiful when you read it out loud; and sometimes it does both. eg:
" . . . an undulant slope of black-tipped white billows, in some parts like a shaken sheet of snow, in others like frozen waves in a waterslide." (page 253)
I love books that cause me to break out my iPad and refer to Wikipedia, google and other resources. This book has caused me to do so some twenty-plus times so far! (45% in.). For subjects such as:
The view from Saturn of the dark side of its rings
The moon Iapetus
Which in turn led me to the discovery of other websites and blogs of interest to me. A nice side-benefit. (although it does extend the the amount of time required to finish the book -- I should have been done by now.)
Not to mention the compelling story of mysterious deaths, conspiracies and evil plots.
So, for an excellent read which delivers far beyond simple enjoyment, but also generates further curiosity, expands my knowledge database, and urges my brain to THINK. Love it.