Hi Herb.!
Ummm - you can't just have "one" book..!!
Seriously - I think you will find Hugh's suggestion a pretty good one. As well - I'd offer Kenyons "Lugers at Random"...
After those, as you narrow down your interests into specific areas, I highly recommend Mr. Still's books. Any one of them - or - all of them...
A great library is a prerequisite to a great collection...
John D.