He's coming in to sign reference book (9)
I believe the answer is:
thesaurus
'reference book' is the definition.
(thesaurus is a kind of reference book)
'he's coming in to sign' is the wordplay.
'coming in to' means one lot of letters goes inside another.
'sign' becomes 'taurus' (Taurus is an example**).
'hes' placed inside 'taurus' is 'THESAURUS'.
(Other definitions for thesaurus that I've seen before include "Wordbook" , "Wordy book" , "Reference book" , "treasury" , "storehouse" .)