While she's not exactly unknown, I consider Elizabeth A. Lynn to be a sadly under-appreciated author. I admit that it's largely on the basis of her dark novel, The Sardonyx Net, but her other books are also strong. When Open Road Media brought out her back catalog as e-books, I bought them up, including this one, which was new to me.
Rather to my surprise, The Silver Horse is not hard-edged SF or dark fantasy, but a short-ish children's story, and a pleasant one. The elements are familiar (transport to a magic world, siblings, rescue, magic, evil sorceress), but Lynn puts them together in a new and satisfying mix. Susannah, the protagonist, is a credible and interesting young teen, and generally a pleasure to follow.
A fun, easy, engaging book for young readers.