I'm no lawyer either but I'd say this is a pretty cut and dried issue:

1. Is contacting the family legally required?  Probably not.
2. Is contacting the family the decent thing to do?  YES!

I would be pissed too if I had a deceased spouse or parent and someone wrote a book about him or her without informing me.  Cr@p, you think they'd at least have sent the family a free copy!