It is just a common symptom of eye fatigue. As we know, long time reading books will cause many problems like vision blur, eye aches, dry eyes etc. So, you need to take regular break when you read looks for the sake of keeping eye health. Now, your blurry vision is just temporary. But you must pay attention to it and let your eyes good rest. Or it may cause nearsightedness or other vision problems for long run.
I think you may have read for a long time. Normally, people are suggested to have a break when reading to avoid eye strain, eye dry and blurring. If it happens frequently, you can buy some eye drops for eye blurring and drip some drops when you feel blurred.