[MattiL]

In addition to overtones, multiphonics are essential part of nearly every Colin Stetson piece that I've heard. So some guidance with them might be helpfull. I do not know about other books, but I have one by Daniel Kientzy that has quite a lot of fingerings for different multiphonics for different saxophones (sopranino,soprano,alto,tenor,baritone).

this is the book i'm talking about: http://www.amazon.com/LES-SONS-MULTIPLES-AUX-SAXOPHONES/dp/B001C0WW80/ref=pd_sim_sbs_b_1