I recently purchased this novel in the Amazon Marketplace. I read it about 15 years ago and no one else ever seemed to have heard of it. (Other than the friend who lent it to me). I couldn't remember who wrote it and eventually I was able to find it here on Amazon, which is one reason I love the internet.
As to the story itself, it starts out a little slowly, as it is unclear who the main character is going to be. Is it Garth? Is it William? We start off with those two but I would say the main character is really Grace Foster, and once she comes along a couple of chapters in, the story gets going. This approach would not fly with today's editors, but if you persevere past the beginning where you're saying "Who is the main character? How is this a fantasy novel?" you'll be glad you did.
I still find this to be an unusual fantasy novel for a few reasons. It's not set in an earlier time or another world. There is a lot of mystery but not "magic" going on, per se. It has a subtle undertone of environmentalism - but it feels natural, not preachy. It takes a satirical tone at times.
And the marmots. Oh the marmots. When I first read this, I thought they were made up. I kept thinking "What is a marmot?" I didn't have a picture of one on the cover like this edition does. Now of course, I know they are a rodent of sorts, like a prairie dog.
One thing I discovered in rereading this gem is that it's part of a series. I'm looking forward to discovering the rest, now that I know it exists.