"Practically everybody in New York has half a mind to write a book, and does"

Groucho Marx's quote is an amusing means of stating that many people in New York have the ambition to compose a book, however few actually follow through with it. He indicates that the idea of writing a publication is so usual that it's almost a cliché. He also implies that the passion to write a book is typically more than the actual initiative took into it. This quote is a humorous way of satirizing the suggestion of writing a publication, and the truth that many people talk about it but few in fact do it. It's a suggestion that passion is not enough to obtain something done, and that hard work as well as dedication are essential to achieve success.