Hey Peter,
I'll throw my hat in the ring too and say that I learned lots from Dans Magnetic Marketing, but have you read some of his other books?
If you like the Ultimate Sales Letter, check out his "Ultimate Marketing Plan" and also his No B.S. series of books too. While they don't go too deep into copywriting, they're all excellent books with many valuable marketing lessons in each.
While I'm at it, I might as well mention Jay Abrahams "Getting all that you can out of all that you've got" book if you want a great marketing education for about $15.
Cheers,
Steve
P.S. If you decide to get Magnetic Marketing, check out eBay first to see if anyone's selling used or new there. I picked my new copy up there for a song a few years back.