Ok, this is probably old news for most of you. But a friend from work who is just getting back into trials after 15 years (with a little encouragement from you-know-who!) brought in a book for me to check out.
Bernie Schreiber's "Observed Trials", copyright 1983. The equipment is older, but the photos are awesome and it's chock-full of techniques, advice, and practice ideas for beginner to advanced riders.
I did a little search, and WhiteHorse Press still has a few copies in stock, so I ordered one for myself. :)
http://www.whitehorsepress.com/product.asp?id=obstr