The object complement is used only with active transitive verbs and is joined directly to the verb without a preposition; it answers the question “what?”
Marco reads (→what?) a book; or “who?”: Lucia met (→who?) her friends.
The object complement can be introduced by one of the forms of the partitive article or by the expressions a little bit of, some, some (alcuni, qualche), we talk about a partitive object complement: I bought some coffee; I found some information; give me some water; lend me some books.