Very fun idea and fun game! It helped a lot after I looked at Help and figured out that I could adjust the books more with my mouse wheel! I was able to get to level 6 (just past the freeze and spikes level), but it was just a tad too frustrating to get any further for me!
The controls are quite good, if a little finicky, and the book stack detection seems quite nice overall! Good project! Music would be a nice addition, but at the same time, stacking books in silence is kind of nice.