Peace,
The word 'kitab' (pl. 'kutub') in the Q has almost always been understood to mean 'a book'.
But I find it odd though, as I am quite sure that all of God's Messengers were given with (via revelation or 'n-z-l') 'messages' ('kutub'). To illustrate this; I am very sure that a book had never fallen onto the roofs of any Messenger's house, and definitely never onto their heads.
We learn from the Q that the messages ('kutub') received by God's Messengers would come piece meal, meaning as and when required/needed by the Messengers, either strictly for themselves or for the people around them.
So logically, to me at least, 'books' were never revealed to God's Messengers. But 'messages' were revealed to them, rather.
I am very sure that a shift in this paradigm will help us very much in our studies of the Q (a compilation of 'messages' revealed to Muhammad). But you may always choose to stick with 'a book' for 'kitab'.
Browse here for word occurance:
http://corpus.quran.com/qurandictionary.jsp?q=ktbHans Wehr: kitab (pl. kutub) piece of writing, record, paper; letter, note, message; document, deed: contract (esp. marriage contract); book.
I post this so that perhaps some of you may want to consider, but you are always free to make a choice.
Cheers!
Mohammed Noh