Short answers, pulled from the story.
Roverandom is a novella about a young dog named Rover who is turned into a toy by an irritable wizard named Mr. Artaxerxes Pam. Rover then travels to the Moon and under the sea to find the wizard and be turned back into a normal-sized dog.
Tolkien wrote Roverandom in 1925 for his son Michael, to amuse him after Michael lost his favourite toy, a little leaden dog, on a beach of grey shingle stones. The toy blended into the stones and disappeared.
Roverandom was first published in 1998, more than sixty years after Tolkien submitted it for publication in 1937, following the success of The Hobbit.
The main characters include Rover (the dog protagonist), Mr. Artaxerxes Pam (the wizard who turns Rover into a toy), Psamathos Psamathides (the chief of all sand-sorcerers), and the Man-in-the-Moon (a friendly wizard who protects people's dreams).
The story's brief glimpse of the Shadowy Seas and Elvenhome links it to Tolkien's Middle-earth legendarium. Traits of the Moon's trees also foreshadow the mallorn trees of Lothlórien.
Roverandom was included in the collection Tales from the Perilous Realm from its 2009 reprinting onwards.