I have all these books and many more, so I have tried to give an appraisal of the ones worth having in 2020 though I avidly collected anything 928 related whenever published or found.

Porsche 928: Alle Modelle von 1977 bis 1995

Buy Direct from 928.org.uk

This is the German Edition! Exists but hard to find in English.

Porsche 928 Essential Buyer's Guide

Buy from Amazon UK

Don't buy a 928 without reading a buyers guide .

Porsche Catalogues

Buy from Amazon UK

Nice book if you like sales brochures