Gomez: I don't know when the Mythos is going to rear its head, as I'm not that terribly familiar with Masks, but my idea is that William has a bit of knowledge. Not any ranks in the skill or anything like that, rather, as he's written a book about the Severn Valley, he might at least recognize some names, that's all. No in-depth knowledge, but he'll likely know Shub-Niggurath is some sort of pagan fertility deity, for instance. What Mythos events he knows of (like the story of the Moon-Lens, or traditions of Byatis and the Stairs to Yuggoth) he'll likely only consider as ghost stories and minor curiosities.
With your approval, of course. I just don't think it would be realistic that he's completely ignorant of things.