“وَ” (wa)a joining word meaning “and.” It connects this word to what came before.Corpus grammarCONJ — prefixed conjunction wa (and).
“كِتَٰبٍ” (kitābin)means “And by (the) Book” in this verse. It is a noun. a naming word. Here it is masculine, not specific, genitive.Corpus grammarN — genitive indefinite noun.