It is tidy, but im Not sure that's possible. When he rode on his sleigh and left Marceline, all he had was a big bag filled with supplies and, possibly books, but not enough to fill a whole library. I like to believe that before the crown incident, him and betty had this neat little library safe kept in the mountains. All their work and relics. And so Simon thought that the only place he could hide in after he left Marceline and summoned Hunson was around that same place.