Today, I made a couple of books trying out the Secret Belgian Binding, which was apparently (I'm sure I read something somewhere rubbishing the claim) re-discovered by Hedi Kyle after several centuries of being A Big Secret (which is odd: give me half an hour with one and I suspect I would have been able to reproduce it. Not sure why it was such a secret).
I used these instructions, which were all I could find online but which I really don't recommend: for example, they don't actually tell you how to work out the thickness of the required spine piece, just how thick it should be and how many pages of how many sections you need. However, they do show the sewing method, which is all you really need: I was a bit apprehensive, as I'd heard it was a tricky structure, but it's not, just a bit tedious and easier with a curved needle.
(I have a memory of reading a discussion of whether one should sew the text block first or make the cover first; the instructions I used did the latter, and as the former method involves sewing onto tapes and then (presumably) pulling them out through the appropriate gaps in the cover, or sewing the cover around the tapes-and-text-block-sewing-thread, it sounds like -- even with being tedious and fiddly -- cover first is less of a pain.)
Anyway, two Secret Belgian bound books:
The Green One
The Pink One
(I didn't like the way, particularly obvious with the thinner threads of the green one, that the cross-spine threads get bunched into pairs by the action of sewing the text block: possibly this is fixed by sewing the text block first, but otherwise I can't see any real way of fixing it other than having an unattractively asymmetrical sewing pattern in the text block.)