First option is a glue stick. It has the advantage of being relatively tidy. The pages dry flat.
Gel medium is the one I've been using and I now realize I can spread it much thinner if I use an old credit card. It also dries flat and can be used as an adhesive and a clear finish. This is not as cost effective as the first options but I do find it kinda multi purpose.
Gesso is a substance that can be used as a glue as well as a substrate that is much like a primer. It dried fairly flat.
I used it to prime a page and glue down another paper. It worked fine but...
The pages curled quite a lot.
The cure for that is to apply more gesso to the back of the curled page.
It certainly solved the problem but I'm wondering if I would have had to do this extra step had I done this with the gel medium. Something more to try. I should also think about the amount of drying time but it is a good idea to do a lot of pages at one time for when those those creative times strike. Glue needs to be spread evenly, giving special attention to the spine as well as the edges.
EDITED: I did do a second trial with the gel medium as a glue. The pages did not curl even though I used the same paper colour wheel as with the gesso. I suppose there are different ways to do various things. The trick is knowing when to use which one!