Books 1-3 are pretty mild, they learn what happened, think about why they got "lost" in space, etc. But in Volume 4, they really start to discover stuff. Especially around the end of Volume 4 and the beginning of Volume 5, there is so much learned in such a short time that you might be confused. I would also recommend trying to read Volumes 4-5 right after each other to help with the confusion.
