In The Rift (Glenraven II)

Kate Beacham had had a bad day. She knew that the pagan religion she had embraced was not acceptable to some. In fact, her parents had disowned her because of it. But she hadn't expected to be assaulted in an alley by three masked bigots, and then return home to find that her beloved stallion had been killed and left in her driveway with a note nailed to his head reading YoU'Re nExt. Kate was too tough a cookie to crumble, so her response was to get out her shotgun and load it. Which was fortunate because next, she found the book....
It was a Fodor-type guide book to a place she had never heard of before called Glenraven. She was positive she had never seen the book before, nor left it on her nightstand. But when she opened it, words magically appeared: Get out of the house, quick.
Bemusedly, still holding the shotgun, she obeyed just in time to see a hole open in midair above her front yard, and a group on horseback gallop out of it, pursued by a nightmarish flying thing that looked like a cross between a dragon and a shark. Good thing she had the shotgun, because otherwise that might have been the end of all hope for Kate, the riders, and both their worlds.
Once before a woman from our world had saved the realm of Glenraven. Now another must do it again - or her own world will be forfeit, too....