Ruth uses a new reference book (9)
I believe the answer is:
thesaurus
'reference book' is the definition.
(thesaurus is a kind of reference book)
'ruth uses a new' is the wordplay.
'new' indicates anagramming the letters.
'ruth'+'uses'+'a'='ruthusesa'
'ruthusesa' with letters rearranged gives 'THESAURUS'.
(Other definitions for thesaurus that I've seen before include "Book listing synonyms" , "treasury" , "Wordy book" , "lists?" , "Volume of synonyms" .)