This is the 135th most frequent Italian word.
quali
"Quali" means "which" or "what" (plural form) in English.
Here, 'quali' is used as an interrogative adjective that agrees with the plural noun 'libri' (books) to ask for specifics.
Quali libri hai scelto per il corso?
Which books did you choose for the course?
In this context, 'quali' functions as a relative pronoun referring to 'documenti' (documents) and introducing a relative clause.
Ecco i documenti sui quali dovrai lavorare.
Here are the documents on which you will have to work.
'Quali' is used as an interrogative pronoun replacing the subject 'canzoni' (songs) while agreeing in number with it.