Hi
Another useful reference is Jefferey Snyder's Romantic Staffordshire
ceramics - good images and cheaper than Petra Williams (although those books
are a fantastic resource to have on hand). The 2 volumes of Coysh and
Henrywood are also standard references. For earlier transfer prints the
Millers Encyclopedia of British Transfer printed pottery patterns has been
useful to me. I also subscribe to to Transferware Collectors Club and find
that really worthwhile.
http://www.transcollectorsclub.org/
Hope that helps.