generally, a rule of thumbs, a good essay would have two VARIED texts for support. if ur set text was a play, then, a film or novel can supplement it very very well.
remember that u dont need to extract everything from supplementary texts. u only need to mention and quote the more important parts, or concentrate on a certain aspect in ur module (e.g. for powerplay, one text might explain the power of the people in great detail, while another is entirely based upon using structure to demonstrate the machinations of powerplay).
also, be brave! go wild! find something that u might enjoy, (many like to use lord of the rings, or the godfather) that has some connection with what ur talking about, and it also makes ur efforts worthwhile and somewhat enjoyable.