Matthew Butterick’s Typography for Lawyers is excellent. It’s the Robin to Bryan “Batman” Garner’s The Elements of Legal Style. It will teach you, once and for all, to use only one space after a period; the difference between an en and em dash; the best fonts to use; among many other great tips and tools.
Although good typography is not essential, it’s still important. Think about it this way: you’ve got a critical argument to make in court tomorrow; do you wear jeans or your best suit? Suit, obviously, but why? Butterick explains typography’s importance best:
I’m not here to tell you that typography is at the core of a lawyer’s work. It’s not. But typography can optimize that work. All writing necessarily involves typography. And good writing is part of good lawyering. So good typography is too. If you ignore typography, you’re ignoring an opportunity to improve both your writing and your advocacy.