Rule book (5)
I believe this clue is a double definition.
'rule' is the first definition.
(I know that order is a type of rule)
'book' is the second definition.
(order book is a kind of book)
(Other definitions for order that I've seen before include "Command harmony" , "eg OBE" , "Kind - sort" , "Ruling" , "Apply for - class" .)