The Holy Spirit in the Old Testament

loads of mystery on this topic, but here are some helps

articles
http://alfredplace.simbahosting.co.uk/pages/sermons/the-holy-spirit/the-holy-spirit-in-the-old-testament.php

http://www.thirdmill.org/answers/answer.asp/category/ot/file/99770.qna

and
This book looks really good, by a great guy

also, on this i'd like to check my copy of Sinclair Ferguson's, The Holy Spirit

Finally, found this quote from a post i wrote back in 2008 when i did 3 sermons on God's Holy Spirit:
(page 59)
"The truth of the Trinity is a New Testament revelation... But should anyone think that according to Scripture God was unipersonal during the Old Testament period and only became tripersonal when Jesus was born, he would be wrong. What is in question here is not the mode of God's being from eternity, but the manner of its revelation in history. I am not saying that the third person of the Godhead did not exist or was not active in Old Testament times; the New Testament writers assure us that He did and was. I am only saying that His distinct personhood is not expressed by the Old Testament writers; though God's triunity is an eternal fact, only through Christ was it made known." --JI Packer, from keeping in step with the Spirit