Holy schmow! Found it! 14 books in! (Well, 13, since I didn't do the Sunday one.)
As a child I numbered these books according to...well, I'm not sure where I got the idea of their order, but I got it mostly wrong. I guess I'm more surprised now that I got this one right.
Anyway, this is another good mix which I think is remarkable, after 15 years. There are visual jokes, puns, character driven stuff, ideas that stretch out into running gags, and it feels really fresh. The one I was looking for was based on Thor having a wheel dealership and (naturally) orienting the humor around the scams that go with selling cars.
I've got another 10 or so books but I'll probably stop for now. Still, hats off to Hart. He did this while also writing jokes for Wizard of Id for a decade. Kinda impressive.