Great idea - and one the Library has included in the various expansion plans over the years. Unfortunately, none of those plans received funding, so we're stuck with the current design, which does not accommodate a drive up book return. With the rising cost of just keeping the building functioning, making major changes to put in a drive up book return is pretty far down on the list.
Interestingly, the original plans for this building included a drive up book drop near the front doors with a curb cut to allow drivers to use it. The Village of Plainfield nixed the plan for the drive up in 1990, citing fear that the drive up book return would block traffic on Lockport Street. The result is the bumped out curb in front of the Library's front doors and no drive up book drop.