Piers Anthony 
Realty Check [EPUB ebook] 

A vacation rental includes a portal to a secret realm in this “intergenerational . . . intergalactic charmer” from the New York Times–bestselling author (Publishers Weekly). Penn and Chandelle, an older couple, rent a house in the city at a bargain price and discover that its back door opens onto an endless forest. Now they know why others were scared off. That’s only the beginning of the oddities about this particular piece of realty. They decide to call in experts: their grandchildren, Lloyd and Llynn, who pitch in with a will, to try to discover what and why. But it may be more of a challenge than they can handle.