The Chicago Public Library used to have a great store, Secondhand Prose, at their outlet in the Lincoln Square neighborhood where books discarded by the system were available cheaply. Many were extra copies of books which had passed their popularity, but some were scholarly works. Later, the store moved to the new Harold Washington Library downtown where it languishes, a ever-paler shadow of its former self.
I picked up Birley's book at Secondhand Prose, the original one. It wasn't the first book I'd read about Roman Britain--maybe the fourth, but it was a good, and well-illustrated, refresher. Birley was known not only as an armchair historian, but also as an archaeologist.