>I have just been given the job of coordinating all the text that appears on
>a new gallery we are developing. To help me to attain quality, consistency,
>suitability for different age groups and so on, I want to produce a style
>manual for contributors.

Roger,
coming from the field of design, I would try sources that are related to
Erik Spiekermann, a German typograph. He has written some books on the
subject of readability and the use of type. At least one of his books is
available in English: "Stop stealing sheep". Online sources for his work
and related topics would be www.metadesign.de or the "fontshop" site (I
don't know the exact adress).
From the Deutsches Museum, Munich I have bought a book about texts in
exhibitions, which is in German I'm afraid ("Texte in Ausstellungen",
Schriften des Rheinischen Museumsamtes Nr. 60).
Good luck,
Monika Gause
mediawerk hamburg (Germany)