“I’m sorry I could not be with you. Today is my day to eat glass,” Dave begins.

Then he brings out Josh Groban to present Top-10 Little Known Songs about Television:

No. 10: “Some where over the rainbow, way up high, there’s a place that has not been home to a ‘CSI',” Groban sings.

No. 6: (to “Climb Every Mountain”): “Check out ‘Big Brother,’ stay for the hour, and if you’re lucky, you might see someone shower.”

No. 2: (to "The Impossible Dream”): “Before you critics craft your reviews, and dole out your 1 to 5 stars, we here at the CBS network invite you to our open bars.”

Letterman interrupts Groban at this point to tell critics if they’re going on vacation after the tour and haven’t yet selected a book to take along, he recommends “Late Night Wars," Volume 18 by Bill Carter from the New York Times.

“It’s a real page-turner,” Letterman says, noting that in this last chapter of the late night story, “Leno tries to get George Lopez to turn (his now ex-wife’s) kidney.”

No. 1: (to “God Bless America”): “Those Cialis and Viagra and Levitra ads you see, pay for your favorite show. All hail E.D!”