Into the Land of the Unicorns question

I cannot find this book, though I've looked for years. I'm positive it is only because I forgot the name of the series, but I read it as a child of about 10. It was about a girl from the 70s or 80s, and she fell through a time portal, and ended up in a land where unicorns were real. I remember a professor who painted his house green to camouflage it, I remember that the leader of the tribe that found her had a translation crystal of some sort, and I remember that she had to sit in the circle of unicorns to be "chosen" by a unicorn. Other than that, I am sadly blank. I remember ENJOYING it, however, and would like to find this series again, or at least know what it WAS...