It is already very functional and i appreciate the time and effort that you have already put into it. I am not that familiar with it yet but if you continue to work on it i would recommend a couple of small things for improvement.

One, when the user hits the space bar standard Mac interfaces usually cause a pause. I would recommend this. Arrows for back, etc.

Also, i would recommend a short menu or screen with copiable or browser call links, with some recommended for library downloads. I quickly found one with Google but more experienced users will have a helpful range of recommendations. Is onlinebooks.library.upenn.edu the best?