Al di meola has written 'picking techniques' but I guess it's out of print... It's based around the up/down strokes, and it is considered as the bible for improving speed and technique...
http://www.amazon.com/exec/obidos/ASIN/079351018X/qid=1019404457/sr=1-2/ref=sr_1_2/102-3609521-1218551

Well, on Amazon, it seems they have it in stock!!