The First Five Pages by Noah Lukeman. Editors aren't reading for fun. They're looking for reasons to reject 90% of what's on their desks. Don't give them a reason to reject yours. Make your work shine, especially those first crucial pages.
Stein on Writing by Sol Stein. The author has some definite opinions on what counts as "real" writing, but the parts about self editing are worth the occasional annoyance.
Self Editing for Fiction Writers by Renni Brown and David King. A complete how-to manual of editing your own writing, an essential skill in today's tight writing market.
How to Write a Damn Good Novel by James Frey. This is one plan for novel writing, suitable for writing adventure fiction of all kinds.
The Complete Idiot's Guide to Publishing Children's Books by Harold Underdown. Get the straight scoop on the children's market from a respected editor.
Zen in the Art of Writing by Ray Bradbury. A delightful series of essays on writing and the writing life, a book I keep turning to again and again.