A Wrinkle in Time by Madeleine L'engel.

Now, it does have a heroine for a main character, and they do ride a creature that has physical similarities to a unicorn, but it's far from girly. Please give it a read.

There are also three other books if I recall, but they are only connected by the main characters and aren't really continuations of the same story. They're all good too, but the first one is the best.