I've read a good few of the Tales of the Slayer and really enjoyed those, I've either forgotten or Haven't read India Cohens one. Must reread those.
Book of Fours isn't cannon is it? I read it years ago and did enjoy it, and I'd read a few of the books, and hadn't liked them, thought the dialogue was trying to hard to sound the same, but I do recall enjoying Book of Fours.
The novels aren't canon but I'm almost positive that Tales is, at least in terms of all the slayers that are featured are acknowledged as being canonically part of the slayer line. India's story is in Vol. III, if memory serves me.