[Ger-Poland-Volhynia] Germans Migrating to Volynia

[Ronald Roggow]

I am curious about Jerry Frank's entry in the Ger-Poland-Volhynia Digest, Vol 83, Issue 10, where he states Catherine the Great's Manifesto had no influence  on the reasons for Germans settling in Volynia.  I had assumed (apparently incorrectly) that her manifesto was the reason for Germans settling in western Russia.
 
Where can I find historical information on why Germans came to settle in Volynia?