You can do no better (in printed terms at least) than Paul Tyers' 'Pottery
in Roman Britain' (Batsford, 1996). It's not exhaustive, but it's the most
up-to-date reference work. For a shorter introduction, there's Guy de la
Bédoyère's 'Pottery in Roman Britain' (Shire, but I'm not sure of the date).
I don't know of any web sites off the top of my head...