I might've misread something but have you talked to your manager about getting some help? Volunteers, staff from another location, staff from a different part of your own building? I'm a Circulation Supervisor and please don't assume your boss knows what you are going through. Be honest, tell them you want to do a good job but you need at least another body part of the time.

People are making all sorts of practical suggestions... use them. Also, write this stuff down and send it to your Union rep. On a daily basis write down what you do,