Stefan Kanfer's Tough Without a Gun is a fairly detailed analysis of what makes Bogie so iconic. If you believe the AFI, Bogart is the greatest American film star of all time. Yes, his acting was rather monotone and his ability to play outside a rather narrow band of characters was slim. But Bogart represents an America that many of us are nostalgic for, hence his staying power.
I found the story of Bogart's youth and his struggle to achieve his fame to be rather amazing. I'd never known that he was so lost -- the directionless son of a wealthy family. Poor little rich kid. Acting was really a trade he fell into, via family friendships and a lack of marketable skills.
I think I'll put on my trench coat, fedora, and go wander around Manhattan.