I built my share of large books for an opera about a year ago, and our solution was a Styrofoam core, lauan covers and halved ethafoam covered with gaff tape for the spine. The foam was split pipe insulation with adhesive along the split. Once cut in half, I used the adhesive to hold the foam while I taped it. we then used single-sided corrugated cardboard
glued to the exposed edges of the styrofoam for the "pages." Our books topped out at about 18" x 24", but the technique may still work for you.
I have luck with muslin covered lauan for the cover and spine, inside made out pink or blue foam, edges textured to look like pages, and painted as you wish. If the raw foam edges give you trouble cover them with muslin after you shape them the way you want. Fairly light weight and durable enough to stand on.
There is a cardboard "honeycomb" that is used in some construction projects that is very light. Applying lauan covers over that, with suitable treatment for the spine and "page edges" would make a very strong prop. Another possibility would be balsa core designed for boatbuilding.
Did a series of over-sized, muslin-covered frames for a dance concert that were joined by multiple hinges. (The frames were 5' x 8', *much* larger than life.) Did some smaller books made of foamcore pages, with lauan covers and a canvas hinge, that worked decently, though the foamcore didn't hold up terribly well. Styrofoam might work as a substitute, if its
edges are protected. Single sheets of lauan might also do the trick -- depends on exactly how lightweight you're going for.