No idea what they used as an adhesive (or whether the paintings were added by someone at Black Sparrow or by the bookbinder), but the only damage I've ever seen is some oil bleed through (from the paint) on the page the painting is attached to. That wouldn't be an issue for the earlier watercolors like this one, or the later acrylics.
The paintings that I have or have seen are all attached to a free page up front with a strip of adhesive along the back left side of the painting. I'll see if I can get a picture later.
I don't know about the watercolors, but the oils/acrylics I've seen always have a sheet of wax paper on top of the painting itself, so it doesn't stick to the page on top of it.
That's a beautiful book at a great price, but is it important in the sense of Buk's career? There seems to be a glut of insanely cool Buk items available at the moment, some big ticket items have definitely shaken loose and there are some I'd own if I had the dough. But are most of them truly important in terms of his career? I'd say not really. They all feel like things that will all eventually show up again, and again.
Also, as Purple said, the interest in these things seems to be waning a bit.
My South of No North has what I would describe as rice paper rather than wax paper as a protector (oy, let's hope this doesn't descend into a materials science diatribe as it did a few years back). This makes some sense as it is thinner and watercolors don't need as much protection - more about the title page than the art itself I suppose.