If you are interested in the (very) early history of Linux, @liw has written an article for LWN which has a subscriber-invite link so you don't have to wait for it to leave the paywall:
Never heard of the en.wikipedia.org/wiki/Sinclair… before, sounds like a cool piece of technology and one of all those branches never followed further.
A real rabbit hole if you click down to peripherals and clones. Very cool for a machine I (and I'm assuming most people) never heard about. How did Torvalds end up with one?