How do I calculate deflection in shelving boards?
Short answer: it will sag a lot. 

Using The Sagulator I get a deflection of about 0.9" at the center, which will clearly be noticeable. (I'm assuming your "5/4 maple" is actually going to end up 1" thick when the finishing is done.)

Also I'm not sure what you are going to be using the shelving for but if this is going to be for books you should probably assume 30 lbs per foot, which would give you a deflection of 1.3".

(You can use that calculator to estimate sag for a variety of shelving configurations, but make sure you pick accurate values for all the inputs. Also note the footnote that says that long-term sag will be 50% higher than the calculated value.)