While the official listing calls this ISBN "Other" I believe that it actually refers to the 1965 paperback edition. This wouldn't make a very good board book! A great ending to a story, full of allusion, metaphor and connections, but also readable just for the story. You finally learn what happened to Ransome after the Silent Planet, and Perelandra the first two books in the series.
Lweis was a master at melding si-fi and theology with social comment. Last book of a trilogy. Not necessary to have read the previous books.
1965 paperback