Well, it seems that your eyes will ocassionally shake which is not a good sign of health condition to be honest. So, there are various types of reasons giving rise to that, such as a lack of sleep, disordered life routine, or something else. Anyway, there are basically no direct relationship with your eyes, but other parts of your body. So you had better take care of your health and find out what went wrong.
According to your description,it may be nystagmus.This symptom can be classified to three kinds---rotatory nystagmus,pendular nystagmus and occupational nystagmus.Therefore,causes are different because of diverse kinds.Motion sickness,sun stroke,eye strain and fatigue are possible reasons.It belongs to compensatory conducts in order to adapt to the changeable environment in and outside your body.so it is ok,don't worry about it.Getting enough rest,then recover ! With best wishes!
When you look at your book carefully, your eye nerves will be in the intense state. That is why your eyes shake sometimes. I suggest you to stop for a while. And have a good rest for your eyes. It will be ok after you have the rest for the eyes. Keep in mind of resting for the eyes after an hour's reading on the reading.
It is normal sometimes that eyes shaking. It is harmless and come and go. According to your description, your shake eyes may caused by tired of eyes or strain eyes. As we know that eye shake is type of spasm of the eyelids. It is can be caused by stress, tiredness, eye strain and too much caffeine intake. So, If you feel eye shake, you must be tiered from reading your book. So, just take a good rest right now.