The title is from Koestler's The Sleepwalkers, and refers to Copernicus's. De Revolutionibus Orbium Coelestium. Historian of Science Gingerich saw that one copy had been well anotated, and so clearly somebody had read it. He then started on a quest to find who had read the early editions, by looking at annotations. This expanded into a search for all existing copies of the 1st and 2nd editions. Combines a biography of Copernicus and his student Rheticus, with an account of Gingerich's studies over more than 30 years.
It is very interesting to see quite how much information Gingerich could find about the lives of scientists from 500 years ago. But I wouldn;t say that it was gripping from start to finish - I sometimes find an account of where the author went and who he met 30 years ago can get a bit tedious.