Lucky me,
I did buy a book on E-Bay which is called:"A Pictorial Record of Great Western Signaling", written by A. Vaughan.
To me, after a quick view, it looks like a bible for signal builders and users. There are plenty of diagrams and b&w pictures from all kinds of projects to modell. Including signalboxes too.
This winter I hope to examine the book carefully and pick some examples to use on my own layout.
I'll keep you posted.
Frits