This is probably not what you want to hear, but I'd only buy a how-to-write book from an author whose writing I knew and liked already. There are tons of how-to books out there by people I haven't heard of, and I have no idea why I'd want to write like them, so I really need a reason to pull the book off the shelf.
So the sff topics I'd want to see would be the ones that I had admired in the author's books, and of course, that'd dependant on the author. Lovecraft's take on crafting believable female characters is nothing I want to read, nor am I particularly interested in his analysis of writing dialog, but I'll re-read a chapter on "Ancient Terror and You" five hundred times. I'd be glued to a chapter on worldbuilding from China Mieville, but I'd snore through something similar from a lot of other authors, even ones I admire. Phillip K. Dick, if he hadn't been crazier than a hoot owl, could have written "Short Story Plots To Kick You In The Teeth" and I would have read it, but I would have torn the chapter on "Making Loveable Characters" out and burned it, and then sown the ashes with salt.
What I want to see in a how-to write book is what the author does that I admire, ya know? So I can't give you any specific advice without having read your work.