+1 on Lopes and Ehwa
Some other highly regarded books:
The Dunhill Briar Pipe - the patent years and after John C Loring also other Dunhill related publications
The Pipe by Geroges Herment
The Pipe Book by William Goldring
Pfeife rauchen Heiko Behrens and Holger Erickert
Pipes: Weber's Guide To Pipes and Pipe Smoking by Carl Weber
The Pipe Companion by David Wright
Pipe: The Art and Lore of a Great Tradition by Robin Crole
and there are books on tobacco, and tobacco smoking, etc.
Hacker did pioneering work, but be aware that much of his information is dated and incorrect. There is much more accurate information to be found on sites like Pipedia and Pipephil as well as later books by others. Hacker's info on Barlings is mixed at best. Still, he's an indispensable read for a general introduction. Just take the particulars with a small mountain of salt.
If you're a serious student of Dunhills and have not read Loring's books, you're not a serious student of Dunhills.