Far be it from me to criticise the 'GOAT' of sentence-smiths... but personally, I'd have deleted 'beastly'. The paraprosdokia (sudden switch in expectations) makes it funnier.
"She spent an hour explaining why she'd written the book, when a simple apology would have sufficed."
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"She spent an hour explaining why she'd written the book, when a simple apology would have sufficed."
In context, though...