I don't know why it's taken me so long to work this out. I've been trying on and off for years to make a photo album I'm happy with... one with pages sturdy enough to support the photos yet not too stiff; with spacers to pad out the pages and accommodate photos without looking unsightly or getting in the way; with a sturdy spine that will withstand the weight of added photos...
I've tried sewing a book with double the amount of pages then cutting alternate ones out but it's hard to get neat and wasteful of paper. I've tried sewing pages to a concertina spine which works as padding but just didn't feel quite stable enough. I've tried folding the edges of pages over and sewing, or gluing, with guards, without and it always came out wonky. To be brief I've tried a lot! And never been quite satisfied.
But yesterday it came to me, just like that. I've been making concertinas from folded single sheets glued back to back for a while (like a line of alternating Vs), it's the Japanese method I think, and suddenly I realised if I pointed all the Vs the same way and put spacers at the spine edge I could glue it all together in the same way, and it would stay straight and square. Turns out it's sturdy with the sort of page thickness I want and the spacers are hidden too.
So, this may be obvious, or just the way it's done, it's probably got a name but I'm just glad I've discovered it. :)