Reference contents of chest in sign (9)
I believe the answer is:
thesaurus
'reference' is the definition.
(I know that thesaurus is a type of wordbook)
'contents of chest in sign' is the wordplay.
'contents of' means to look at the middle letters (the letters on the inside of the word).
'in' means one lot of letters goes inside another.
'sign' becomes 'taurus' (Taurus is an example).
The central letters of 'chest' are 'hes'.
'hes' placed within 'taurus' is 'THESAURUS'.
(Other definitions for thesaurus that I've seen before include "lists?" , "treasury" , "Word-finder" , "Dictionary" , "Book listing synonyms" .)