The Otherworld is an omnibus edition of the third and fourth books in the SERRAted Edge series - When the Bough Breaks and Wheels of Fire.
There are elves out there. And, mostly, they're the good guys. Oh, they drive too fast (they love hot cars) but we'll forgive them that since they also make a habit of rescuing children in peril.
But this time it isn't just the children who are in peril. Jamie's father has joined a fanatical cult that is in contact with a supernatural entity that is ancient, powerful- and unspeakably evil. And Amanda is an abused little girl who has unheard of psychic powers. In her pain, she might last out and destroy Earth and Faerie both!
The Bedlam Bards series, the SERRAted Edge series, and the Diana Tregard books are all set in the same universe, and feature elves, magic, and modern life.