Often recommended, but Anne Lamott's Bird by Bird and Stephen King's On Writing are both autobiographies wrapped in textbook sheepskin. Writing advice that may seem obvious or trite is then backed up with beautiful vignettes.

Good writing is not necessarily good speech, as you've noted. Learning technical writing is a superpower — I would recommend The Essentials of Technical Communication. A chemistry teacher once introduced it alongside a Vonnegut quote:

"It would never occur to me, to look for the best minds of my generation in an undergraduate English department anywhere. I would certainly try the physics department or the music department first."

Technical writing and writing for emotional impact both have their place. Learn both, and learn when to employ them. You may find yourself appreciating that Thoreau quote one day.