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You mustn't judge a book by its cover.
Literal
Book [object-を] that cover [by-で] judge-[forbidden-てはいけない].
A version of the 'don't judge a book by its cover' proverb. The prohibitive ~てはいけない is used — softer and more instructional than the bare ~な prohibitive imperative. その refers anaphorically to 本's own cover ('its cover').