Rough Sleepers by Tracy Kidder

 Rough Sleepers describes the lifelong mission of Dr Jim O'Connell to aid the homeless people (the rough sleepers) of Boston.  He started his work as a favor to one of his bosses at Massachusetts General Hospital, where he was completing his residency. But then he realized that this is what he was meant to do with his life.

It's an inspirational book and one that makes you appreciate your own blessings.