The City on the Edge of Forever: The Original Teleplay

If you're Harlan's fan you'll enjoy this, especially if you like/don't mind his rants. Because there's a loooong one in this book. And it's well justified.
If you're a trekkie, don't even bother. You'll be butthurt - as is evident by the one and two star reviews - because Harlan exposes Roddenberry's lies and shows how the stuff was done behind the scenes.