The Nexus Ring — Maureen Bush

Maddy's ring appears to be an ordinary stone ring, but soon, as the family travels through the mountains of BC, a menacing man, Gronvald, tries to steal it, and an elegant woman, Aleena, offers to help them keep it from him.

Aleena takes Josh and Maddy through a veil of magic, to a wondrous world that coexists with the human world. But Aleena can't be trusted―she covets the ring for herself. The kids then meet the otter-people, who insist the ring must be taken to the giant of Castle Mountain for safekeeping, so it can no longer damage the veil between the worlds. Somehow Josh and Maddy must return the ring to the giant. And then try to find their way home.