The problem might be with the publisher rather than with the quality of the book - Arnoldsche also mucked about with a series of three books on WMF. Volumes 1 & 2 were published, and are both excellent texts, but Vol. 3 vanished into the ether after a few promised-but-unfulfilled release dates.
If, for some reason, it does only get sold through venues hosting the exhibition, it might become available through the individual venue websites. I've managed to get a few exhibition-only books from various galleries' online shops when I've not had chance to visit them in person.