When I quit my last job I gave just over a month's notice, b/c I knew they would want me to help find my replacement. Unless you work in a cuthroat, competitive environment (I can't imagine a library is that way) I wouldn't worry about it. People are always worried about how to quit, how their boss will react.....unless it's extremely short notice, I've never been mad at an employee that quit. We always want our employees to succeed, and if that means going back to school, then so be it. Granted, I work in higher education and we generally give 3-6 months notice when possible, because our job searches run on academic year cycles.