My favorite Peanuts book has always been this one, if only because of the title and the bizarre...well...NON-appearnce of the character, Thompson. Apparently a secret operative working for the Head Beagle, with whom Snoopy seems quite familiar, Thompson is, one assumes, a dog.
Snoopy receives a coded message saying "Thompson is in trouble." and he's off and running, trying to tack down the missing agent by disguising himself and interrogating a cute waitress. Eventually, though, our hero arrives too late and Thompson meets a gruesome end.
This uncharacteristic, non sequiter of a storyline ran for 10 days circa 1972.