From Evolution: Education and Outreach comes a nice introduction to the concept of homology.
Late note: OK, not so good. I saw only the cute pix, and presumed the author understood paraphyly. But as I didn’t have an entirely functional computer at the time I leapt in too soon. See Malte Ebach’s and David Williams’ takedown here. Their point is that paraphyletic groups are formed by what hasn’t evolved, and so they are fundamentally antievolutionary groupings.