When I return a book I put it in a bin, and a (non-Jewish) worker later takes it from the bin, and scans it into the computer as "returned." As such, can I return a library book on Shabbat?
Assuming that you know 100% a non-Jew will be on duty, and assuming that scanning books is forbidden on Shabbat.
I think it would depend:
If you have to have the book returned by Shabbat - or else incur a fine - then you are essentially asking the non-Jew to process your book now.
Asking, hinting or otherwise getting a non-Jew to do work for you on Shabbat is forbidden.
If you couldn't care less if the book is marked as returned on Shabbat, or on the next working-day, then you are not asking the non-Jew to work for you on Shabbat.
Unless the non-Jew will get into some trouble for not processing your book immediately, there shouldn't be a problem returning the book.
That said, it sure doesn't seem Shabbas-dik to be returning books on Shabbat.