Good question. I take it to mean the calendar is on top of the book, "upon the book." I think if a person wanted to communicate that there's one printed on the face of the book one would want to use a different preposition, such as ב, or lengthen the prepositional phrase to something such as על כריכת הספר, "on the cover of the book." [Thanks, chantal, for the encouragement. Have a lingot.]
The word לוח in Mikra can refer to a board or plank (wooden planks used for a door in Song 8:9) and is famously used to refer to the stone tablets associated with Moshe. It's the same word in Aramaic and Syriac. Since one can write on a לוח, people used them as "writing tablets." The Greeks and Romans called them pinax. They could be made of wood, but wood tends not to survive from antiquity. So for לוח one can picture something to write on; one way to look at לוח שנה as "something to write the year on."