The Scribes Journal of Legal Writing has published a volume of transcripts of interviews with US Supreme Court Justices on legal writings. Conducted by Bryan Garner (the founding editor of the Scribes Journal), these transcripts were drawn from video interviews with the Justics from 2006–2007. It includes interviews with
John G. Roberts, Jr.
John Paul Stevens
Antonin Scalia
Anthony M. Kennedy
Ruth Bader Ginsburg
Stephen G. Breyer
Samuel A. Alito Jr.
Themes of the transcripts are “clarity and simplicity, honesty and accuracy, overlong briefs (and opinions), rewriting and re-rewriting, attending to grammar, anticipating the other side’s arguments, the primary importance of briefs in decision-making, and the professional need to cultivate strong writing skills” (from the introduction).
Interestingly, Justice Souter is was the only one to decline (Sotomayor and Kagan had not yet been appointed to the bench). Souter wrote, “I’ve never been satisfied with my own prose…Since I don’t think my own work is worth writing home about, I’d feel presumptuous telling other people what they ought to do.”
During my work as a legal researcher and writer, I have found that much of the writing I encounter is atrocious (with the caveat that my observations are incidental to my particular work). I should note that most of those writings are at the trial court level, which makes a difference, of course. But I have also have the idea that law schools teach students how to write legal documents only as technicalities, not as art. I believe that those of us involved in the writing professions ought to continually learn to be better writers. (My critics might rightly point out that, as a editor and writer of 20+ years,  I have a bias.) This volume is a wonderful source of ideas about legal writing. Of course, some of the observations are merely opinions or pet peeves, but that does not lessen the value (especially considering the source).
Quotes from the volume:

“Every lawsuit is a story. I don’t care if it’s about a dry contract interpretation; you’ve got two people who want to accomplish something, and they’re coming together — that’s a story. And you’ve got to tell a good story.” (Roberts)

“…it’s perhaps unfair, but if someone uses improper grammar, you begin to think, well, maybe the person isn’t as careful about his work, or his or her work, as he or she should be if he doesn’t speak carefully. Grammar is really quite important. And we don’t encounter grammatical errors too often.” (Scalia)
““I have yet to put down a brief and say, ‘I wish that had been longer’.” (Roberts)
A PDF of the volume is here (192 pages).
The video interviews are available on Garner’s website, Law Prose.
(Garner had a number of discussions about legal writing with Scalia while trying to convince him to allow an interview. Those discussions led to a book co-authored by them: Making Your Case: The Art of Persuading Judges.)