Two short essays -- of which the first (The Writer as Detective Hero) contains some real gems of advice; the second, on writing the Galton Case, is less interesting.
I haven't read any MacDonald -- but this book has made me want to.
One point RMacD. makes is that the first "mature" book he wrote -- breaking free of the conventions of hardboiled narrator/hero -- was the Doomsters. From then on, he was a different writer (he says).