Strunk and White's "The Elements of Style" changed my writing (and maybe my life). I call it a hornbook for all writers. I applied its principles to my writing while working for my community college newspaper for several semesters. Elements helped me with conciseness (though I would still try to sneak in the Oxford comma because I believe such always adds clarity and balance).
Stephen King's "On Writing," King says to "Kill your darlings"; I say you have to kill your characters (meaning the alphabet kind). "On Writing" also helped me chuck 99% of my adverbs because nothing beats "s/he said" or "s/he asked." You want dialogue tags to be invisible.
"Self-Editing for Fiction Writers, Second Edition: How to Edit Yourself Into Print" by Renni Browne and Dave King made me a better creative writer—I went from telling a story to showing one through scenes and dialogue. By applying the principles in this book to my novels and short stories, my characters came alive.
I credit these three books and my experience as a student reporter in helping me get work as a professional editor.