Otherland, book 2 : River of blue fire
Tad Williams1998

Otherland. In many ways it is humankind's most stunning achievement: a private, multidimensional universe built over two generations by the greatest minds of the twenty-first century. But this most exclusive of places is also one of the world's best kept secrets, created and controlled by an organization made up of the world's most powerful and ruthless individuals, a private cartel known - to those who know of their existence at all - as The Grail Brotherhood. And now a small band of adventurers have penetrated the veil of secrecy that prevents the uninitiated from entering Otherland. But having broken into the amazing worlds within worlds that make up this universe, they are trapped, unable to escape back to their own flesh-and-blood bodies in the real world.