Do you know the name of this book about otters?

I read a book when I was younger (it's probably a "young adult" fantasy book) about a lady who, when she sees bright, reflective light, gets drawn into an alternate, magical universe. She is taken in by two mentors: an old lady and an anthropomorphic talking otter who she kind of falls in love with. They teach her magic, and they are trying to fight some evil force - I forget what the evil force was. On the cover, there was a picture of the otter floating in space and reaching for a bright light (like a star or something). Any ideas???