The best sources on legal writing--a series
This is, if not the best, certainly one of the best books on legal writing:
The Lawyer's Guide to Writing Well by Goldstein and Lieberman
It changed the way I think about lawyers as writers. It made me realize that lawyers are professional writers and that as a professional writer, I should take the craft of writing more seriously.
That I now read everything I can about legal writing, that I now look things up instead of relying on what I learned in 8th grade, and that I now view legal writing as an independent field of study and not just a "skill," all these I owe to this book.