as with the above, I suspect this has more to do with the surface texture from the mold than the inherent frictive properties of the material. Nevertheless, it is, perhaps, more important to measure that, so people will know what to expect when their new keycaps arrive.
I suppose to quantify the coefficient of friction (or even qualify it more accurately) one should cast a block of it, then machine it to a standard mass and surface area (of the sliding surface) to get as uniform a surface as possible.
Some polymers, such as POM (delrin) absolutely have lower coefficients of friction, which is why I prefer POM spacebars. Nevertheless, I am certainly inclined to believe the graphic posted in your textbook.
Now friction is a rather difficult thing: you see it's based on a number of factors (there's some sort of inherent baseline, then there's surface roughness, and porosity, etc.) I want to see the porosity of PBT compared to ABS, as I suspect PBT to be more porous, and hence better for dye sublimation.
Come to think of it, it makes sense that PBT has lower friction. Has anyone felt the side of a unicomp keycap? The sides are smooth, and it really shows.
Excellent writeup and I'm glad you posted.