I have shoulder pain because I have to read a lot at work. What can I do?

Ergonomics. Have an occupational specialist eval your set up at work, including chair, keyboard, mouse, monitor level, accessory tools, etc. Make sure all of your work, including reading, is within the ergonomic "box". Also consider physical therapy for exercise recommendations. Hope this helps!
Do not lean on elbow. The ergonomics of your job place is important, do not lean on your elbows hard.
Change positions. Supporting a weight even a small one for long periods of time can lead to joint pains and stiffness. Changing positions stretching and breaks will help.