Somewhat more serious, and somewhat more obscure, but a series I enjoyed a lot growing up is the "Pelbar" (or Pelbarr) cycle by Paul O. Williams. Kind of old though, early 80s, and probably out of print.
Thanks for all the great suggestions! I've updated the initial post as a summary. This is actually reminding me of a few P-A books I read in high school... now if I could just remember the titles....