I've used Monroes sequence for years, I've always been surprised it isn't talked about more in copy circles - but I'm guilty of keeping my mouth shut as well
Another book to spur the free flow of words onto paper -
Writing the Wave: Inspired Rides for Aspiring Writers by Elizabeth Ayres
Great at blasting away writers block - many of the exercises can be converted from creative to copywriting.
And another writer's-block killer...
Writing the Natural Way by Gabriele Lusser Rico
Great exercises to free your mind.
One Jay Abraham recomended to me back in the early 90's that I really enjoyed was...
How to write, speak, and think more effectively by Rudolf Flesch
Great book for writing simply, with power. In fact, any of Flesch's books are great reads.