It's a book, lady (3)
I believe the answer is:
she
This clue is a triple definition!
'it's' is the first definition.
(thesaurus)
'a book' is the second definition.
(I've seen this before)
'lady' is the third definition.
('she' refers to a woman)
(Other definitions for she that I've seen before include "Female person" , "fiction" , "lady" , "The one who must be obeyed" , "Book" .)