Hour of the Horde by Gordon R. Dickson: Against an approaching merciless alien horde is an outnumbered galaxy-wide legion.
Wolfling by Gordon R. Dickson: Earth was only a primitive outpost, its people dubbed primitive "wolflings" by the galaxy's rulers -- until Jim Kell was sent to the rulers' Throne World.
Gremlins, Go Home by Gordon R. Dickson and Ben Bova: Suppose that elves, gremlins, and leprechauns are really aliens marooned on Earth centuries ago. Now they can go home -- if they can hijack NASA's Mars rocket!