Exactly! So when the book says "be concerned if an hour has passed since the water bag appeared, with no further progress", does that mean the clear fluid filled bag, or the lamb bag!? (presumably the lamb bag?)
The lamb is
in the lamb bag, so it's an hour after the water bag - the clear fluid-filled one. But you won't always see that, or it may have burst and all you see is a string of matter.
The hour is of course just a guide. When you've seen hundreds, as I now have, or many thousands, as BH has, you start to develop a 'feeling' for when something's amiss, and may intervene sooner, or leave it longer. And you learn to never ignore your intuition

If you keep everything clean, use plenty of lube, are
gentle, and have the ewes indoors anyway, you won't do any harm by having a feel too early. So for a novice, an hour after the water bag, or after the first for a twin, or 20 minutes after the previous for triplets, is a good guide. Soon enough to rescue most problems and not so soon that you'll be sticking a hand up every ewe.
When I say 'have a feel', I mean an explore with your washed, gloved, lubed fingers. If you can feel a lamb, then explore what you've got, and if it's two front feet and a nose, you might decide to give her another 30 minutes, or just get on and help her deliver it. If you can't feel anything at all, don't go rummaging around past the cervix (which is likely to be quite tight still, anyway, if there's nothing coming.). Either give her a while longer, or if you're worried that she's tiring / giving up, get the vet or someone more experienced than yourself.