Your observation about writers doing well on the LSAT is very interesting to me, Mike. I was one of those LSAT “naturals” (I always feel guilty when my students ask me what I did to prepare; the honest answer is “Not much,” but do I as say, not as I did), and I am also a writer, and was one long before I thought seriously about going to law school. I was an English major, I worked full-time in the print journalism field, and I’ve written numerous short stories, and have a couple of unfinished novels and a book on boxing lying around waiting for me to dust them off. But I never correlated LSAT success with writing; I always attributed it to other things I did that had a more direct connection to formal logic – specifically, my years as a competitive bridge and poker player.
Like you, I’m very impressed with Stephen King’s “On Writing.” People who aren’t familiar with King and dismissive him as “Just a horror writer” are often surprised to learn that he wrote the novella that became (with pretty much no changes) “The Shawshank Redemption,” to say nothing of “The Body” (which became the outstanding movie “Stand By Me”) and “The Green Mile.” Not all great writers are great teachers, though; King is. He’s not just written a bunch, he’s obviously spent a lot of time thinking about the craft of writing, and he laid it all out there in “On Writing.” I reread it annually.
With your correlation in mind, Francine Prose’s “Reading Like a Writer” offers a very insightful look at the relationship between reading and writing, and will IMO make anyone who reads it much better at both.