Not sure if i understand your problem completely, but i'd like to comment on a possible side-issue of your design. Does the time-limit thing even make sense to the user?

Say your limit is 30 seconds. Then the user is forced to handle books quickly enough to be able to meet that limit. On the flip side, when the user wants to finish, they have to wait the 30 seconds twiddling their thumbs.

Wouldn't it make more sense to have i "i am finished" barcode stuck to the place where the scanner is mounted, as well as one for "cancel my last scan" and "cancel the list" each?