"Mi amigo lee muchos libros."
Translation:My friend reads a lot of books.
Late I know but I'm wondering why the question about a pronoun. Perhaps I'm overlooking something but I have an example here of 'mi' as a determiner or type of adjective just to clarify.
Possessive determiners are a type of adjective, the equivalent of such English words as "my" and "your." Like other adjectives, they need to match the noun they refer to in number and gender. Here are the possessive determiners of Spanish along with a sample sentence for each:Mi, mis (my, mine): Mi gato es muy peludo. My cat is very hairy.