In the Early Days, a Program Book (Programme Book in the UK) was a publication distributed at registration at a convention which listed the convention's program schedule and other useful information (such as, perhaps, biographies of the GoH).
As conventions grew larger and the program expanded, it became difficult get the program finalized far enough in advance of the con to permit inexpensive printing of the program book, and too expensive to print it once the program was final. Additionally, the program book (now often containing articles, ads and even sometimes fiction) was too cumbersome to conveniently carry through the whole convention. Conventions responded by printing a Pocket Program at the last minute using inexpensive repro technology and lesser typographical values than the (often fancy) program book.
Since the 80s, it is normal for conventions to issue two publications at the door: a Pocket Program (which sometimes no longer fits in a pocket) and a Program Book (which no longer contains the program). Starting with Noreascon Three, some conventions have begun calling the Program Book the Souvenir Book which, perhaps, better reflects its new function. Some cons are now producing them after the con, with photographs and reflections on the event, even the final accounts.