I highly recommend The Science Behind the Art of Legal Writing, an excellent book by Catherine J. Cameron and Lance N. Long, which just came out in a second edition. They’ve taken some truisms and common teaching  points from legal writing and done the research to see if those truisms and teaching points can be validated with science. For example—

• Does format matter?
• Why is passive voice so hated?
• Citation—does anybody really care?

They also address—

• macro- and micro-organization
• the use of narrative
• readability and plain language
• what persuades and what doesn’t

It’s easy to read and full of good advice.