I've run across others online, in books too, but at the moment I am just drawing a complete blank. Sorry.
well therei's the Homeschooling for Excellence books. I can't remember the names but google should turn something up. They were a family of political hermits in the 70s who pretty much unschooled their kids while running a farm high up a mountain and having literally no money. I think basically they were too busy to interfere much in their kids' educations. Of their four kids, (two adopted) , three went to Harvard, iirc, to do law/medicine and the other I believe went to uni too-I think he was still at home when the books were written? The title is a bit tongue in cheek, I think.
Oh to add. To me unschooling is about accepting your kids and how they are, and that they might not end up academic sucesses. Really not. Noticed you are in the UK, OP? I think unschooling is extremely common among secular families, esp with older kids, but I also think we have specific issues around our education system that from everything I've read don't apply in the US. Are you currently a homeschooler? I'd be surprised if there weren't secular/autonomous groups in Cambridge.
Uh oh. Bad sign. I'll just say, "everywhere where you are". It's not really everywhere, is it? Aack!