Moo, Baa, La La La by Sandra Boynton
One of the greatest books I've ever read. It's educational (sounds animals make), silly ("three singing pigs"--genius!), and open-ended ("what do you say?"). It even has ambiguity, both in its open-ended-ness and, more importantly, in the representations of the characters. Boynton's art style offers animals whose attitudes are far from clear. What might look like sadness on the surface might, upon further contemplation, suggest despair or confusion. A face alight with glee might also be one of manic loss-of-control. A pig uttering its species-appropriate "oink" looks strangely disgruntled, perhaps even filled with longing for the days of show-biz.
What more could you ask from great literature?