It’s not a book on Dharma, but on philosophy. In the second half of the book, the “Persons” part, the author tries to determine, using rigorous logic and imaginative thought experiments, exactly what can be discovered about what makes a person a person, and what makes a person in the present the same person as they were a year ago.
Here is a BBC special that was based on the book: http://www.youtube.com/watch?v=34N3pbysIzQ
I find the book very challenging to read, but fascinating nonetheless. Although Parfit is not a Buddhist and was not familiar with Buddhism when he wrote the book, there is a striking similarity between his conclusions and the Buddhas teachings on Self.