Reference book shows Zodiac sign he's within (9)
I believe the answer is:
thesaurus
'reference book' is the definition.
(thesaurus is a kind of reference book)
'zodiac sign he's within' is the wordplay.
'zodiac sign' becomes 'taurus' (I've seen this before).
'within' indicates putting letters inside.
'taurus' going around 'hes' is 'THESAURUS'.
'shows' acts as a link.
(Other definitions for thesaurus that I've seen before include "lists?" , "Book listing synonyms" , "Wordy book" , "Word book" , "Wordy gathering" .)