I really don't get it by reading weird lines.
Suddenly the book has legs and Cavendish is rolling in money from royalties.
What does it mean by "book has legs"?
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Macmillan has a dictionary entry that explains this quite well:
legs (pl. n.) the ability of a product, advertisement, idea, book etc. to continue to be successful and interest people for a long time
So it means the book had sudden success and popularity. I suppose one could say something similar about Rowling's Harry Potter series.
I can see why this would be tricky if you aren't familiar with the expression. It's a figurative expression; the idea is that legs can carry something for some distance.