The concept of a Source Book is not one limited to this sector. From the bespoke books sector (a firm called ubyu) offers another good intro to the Source Book concept, please see quote below which I trust is illustrative:
'You know, when we set about creating ubyu we realised something was missing in the world of self-published photobooks. It was not complicated. It was very simple in fact. What we found ourselves being asked time and time again was this: how do I know which book to order? It actually went a little further than that to questions like: where can I see the papers? How do I know the headbands are what I am looking for? and what does your print look like?
So we put our thinking caps on and decided the best way to meet these needs was to create a little book that gives you everything you want to know. We called it the SourceBook. It has been immensely popular. So popular in fact that we have launched the second version for you to order. '
Daniel G: The idea for this EI Source Book is basically the same - to be a great starting and reference point for enquiry.
We hope that this is useful.
Daniel Gilbert, EI Source Book