Since he's actually a Nazi assassin from Switzerland, we're going to guess that his real name isn't "Joe Cinnadella." "Joe Cinnadella" is just the fake name he uses when he's pretending to be an Italian truck driver. Under that name, Joe ends up in Canon City, Colorado, where he picks up with Juliana Frink. And as Joe, he proposes a trip to visit their new favorite author, Hawthorne Abendsen—but his real reason for going is to kill Abendsen, who has a more awesome name.
So, we actually know very little about Joe—and, in fact, sometimes his lies seem to be partly true. For instance, he talks about his experience on the Italian side in Africa during WWII, fighting against the British (6.49). Eventually, though, he confesses that he fought in Africa during WWII against the British, but with the German army (9.63). If this second story is the truth, does that little difference matter?
But is this second story the truth or is he lying still? We don't really have any way of finding out. (We've tried Google, but there's no info there, probably because he's a fictional character.) Much like Frank Frink (original name "Fink") and Baynes, "Joe Cinnadella" is a bit of a fake person. Boy, Juliana Frink sure has a thing for people with false names. (See our "Theme: Lies and Deceit" section for more on that.)
Whatever his actual story is, Joe Cinnadella plays a big role in the spy and arts plot (check out our "Brief Summary"): he gives Juliana the book, The Grasshopper Lies Heavy, and the idea to go visit Abendsen. So, even though he's a lying assassin, he helps put Juliana in position to gain her enlightenment. Funny how these things work out.