Spaceships, Aliens, and Robots You Can Draw (Ready, Set, Draw! by Nicole Brecke (2010)
How to Draw and Save Your Planet from Alien Invasion! by Sheldon Cohen (2011)
"This humorous guide uses a cartoonish style and clever captions to show kids how to draw and defend Earth from intergalactic invaders."
1-2-3 Draw Cartoon Aliens and Space Stuff by Steve Barr (2003)
One of a series of books. See his website for sample pages.
How To Make Comic Strips
Brian Zaikowski, creator of Demented Denizens comics, has a dowloadable training video set: How to Draw Ancient Space Aliens. He also has lots of alien cartoons you can enjoy on his website. Click here to visit his website.