The book Reese leafs through at Finch's place is 'The Ghost in the Machine' by Arthur Koestler. 'Ghost in the Machine' is a phrase coined by the Oxford philosopher Gilbert Ryle to describe René Descartes' mind-body dualism relationship. In the case of Person of Interest's arc plot, Koestler's book subtly hints that The Machine may be developing a soul, or already has. The soul is famously called 'Ghost in the Shell' in the Japanese manga and anime classic by Shirow Masamune.
Number crunch refers to the process of doing large calculations. In addition, crunch is a colloquialism for trying to manage a big task, often under pressure and with very little time. In this context, the title refers to the larger than usual number of persons of interest the Machine has sent the team, and the challenge that comes from trying to protect all four at once before while finding the threat. There may also be an element of the physical impact of the car crash which sets events in motion.