Plant book (6)
I believe the answer is:
annual
I believe this is a double definition.
'plant' is the first definition.
(annual is a type of plant)
'book' is the second definition.
(annual is a kind of book)
(Other definitions for annual that I've seen before include "plant > book" , "Yearly publication" , "Type of year book" , "Plant that lives and dies within one year" , "Book; occurring once a year" .)