I did a presentation on this a couple of yrs ago, so you're in luck. Not the most up to date, but try these:

Bowler SD, Green A and Mitchell CA. Buteyko Breathing techniques in asthma: a blinded randomized controlled trial. Medical Journal Australia 1998; 169:575-579

Buteyko Breathing Centre. Welcome to the Buteyko Breathing Centre

Cooper S, Oborne J, Newton S, Harrison V, Thomson Coon J, Lewis S and Tattersfield A. Effects of two breathing exercises (Buteyko and pranayama) in asthma: randomized controlled trial. Thorax 2003; 58:674-679

Green, Brightling, Pavord & Wardlaw. Management of Asthma in Adults: Current Threapy and Future Directions. Postgrad Med J 2003; 79: 259-267

Holloway E, Ram FS. Breathing exercises for asthma. Cochrane Database
Syst Rev 2001;(3):CD001277.

Thomas M, McKinley RK, Freeman E, Foy C, Prodger P, Price D. Breathing retraining for dysfunctional breathing in asthma: a randomized controlled trial. Thorax 2003; 58:110-115