Classic Fantasy with characters from the real world

I read this book about 6-8 years ago, and it was already slightly old at that point. The key parts were that the characters were all from Earth (America, if I remember correctly - one character internally monologues about remembering exactly how many steps it takes to get to a specific beach from his house, and while I don't remember the beach name, I remember thinking it was American) and woke up in a fantasy world, scattered about, and not remembering where they come from for quite a while.

Each abductee is wearing a bracelet with (what I remember as) polyhedral dice, although they aren't named that I think I inferred it from the description, and at some point they start using them as a form of magic. They find out that they were all abducted by a wizard so they go to find him. I can't remember what happens next, other than that eventually they find him and in a later book in the series (that I only saw the blurb of) they go back to his castle.

I think the book might be called "______ Keep", and am fairly certain the later book in the series is called "Return to ______ Keep".

I'd really appreciate it if someone who knew the title could tell me - it's like an unscratched itch at the moment, and I'd love to read it again!