Hmm.. I don't think Billy ever imprinted because he never turned into a werewolf. The Cullen's only arrived a couple of years before Bella did in Forks so that would have made Billy too old/too sick.

In a personal correspondence with Stephenie (it's over on the lex) she said that Jake's mom died in an uneventful way, so there's not really a story there.

Maybe her death might have something to do with the next book ...

who really knows
it's one of those little things that you always wonder and don't really find much about.
[[e.g. the vail ;D]]

Neh! She was never mentioned before, and Jake doesn't seem to notice life without her so she must have died when he was very young. I don't think she'll be an important topic at all. Maybe later in the series, if Jake has a book.