Reading through the Preact source, I came across this short introduction to JSX. It’s clear and does a good job of explaining how JSX works. Recommended.
(I’ve avoided the newish JS frameworks over the past few years since things were changing a little too fast to make it worthwhile for me, but React seems to be here to stay, so I’m diving into it when I have free time. (We’re also starting to use it at work.) FWIW, I did look at Angular and Vue, but neither really appeals to me.)