I won't quibble over such fannish preoccupations as why Harpo's hair is blond on the cover or why Zeppo is not there at all; nor more serious concerns such as that this work has no index (yes, it does need one). My biggest complaint is that this book does not finally and formally end the long-lived and oft-repeated falsehood that the song most associated with Chico Marx (the song he "can't think of the ending" of in Animal Crackers, that is the main theme and is played by Harpo in Monkey Business, and is Chico's motif in Horse Feathers) is "Sugartime" (which itself is often misidentified by its first line, "Sugar in the Morning").
The song in question is, "I'm Daffy Over You" written by Chico and Sol Violinsky (Sol Ginsberg). "Sugartime" was written by Charlie Phillips and Odis Echols (born in 1937 and 1930, respectively) and published in 1957, and was a huge hit for the McGuire Sisters in 1958. Yahn lists "I'm Daffy Over You" in connection with a radio program and sheet music, but does not identify it with any film. He clearly misidentifies the song several times as, "Sugartime."
That this book, ostensibly focusing on the Marxes' music, does not address this issue is shameful, particularly since the matter has been resolved on Marx Brothers listservs and websites (for instance, see Stefan Timphus' magnificent site at marx-brothers dot org [Amazon won't allow URLs, y'know]; you can even find the lyrics to "I'm Daffy Over You" there in the Media Files > MIDI Files).
How reliable is the balance of Yahn's research?