Wollheim, Donald A. The Secret of the Ninth Planet. Philadelphia: John C. Winston, 1959. First edition. Fine in a price-clipped first state dustwrapper with old tape reinforcement to top edge.

An early work by the great editor, published as part of a charming and much beloved series of science fiction juveniles published by Winston.  Most of the books in the series went straight to libraries, and kids read the rest to pieces, so copies in such fine condition are very uncommon.