What does the cryptic crossword clue "Ruth uses a different reference book (9)" mean?

The definition is "book". The wordplay uses an anagram. anagram of RUTHUSESA ("Ruth uses a") [anagram: "different"] = THESAURUS; definition: "reference book" The answer is THESAURUS.

Ruth uses a different reference book (9)

Guardian Cryptic #23996 | 14 Down | 2007-02-09 flower

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