The Window to Tellkar - J. S. Frankel - Bog

Windows show us the world outside. As Mark Cornish discovers, his classmate Glenda can create windows to other realms-not all of them friendly.
A window offers people a view of the world outside. For Mark Cornish, seventeen, a high schooler with a wayward single mother and someone with no prospects, windows offer an escape from reality.
That escape comes through the person of Glenda Cron, a classmate of his with an unusual ability. She can open windows-gateways-to other worlds. And she isn't human, not exactly. She comes from Tellkar, a world somewhere in another galaxy, a world of shape-shifting, magic and various abilities.
It seems that Glenda-real name, Glynarra-is wanted by one of the most powerful kings on her world. King Lensa wants to acquire her powers, and when she won't transfer them to him-which would mean her death-he places a curse upon her head. If she travels to another realm, she can only stay a maximum of three days.
Mark agrees to help her, even though he has no powers and no special abilities, save one. He won't give up. As the two journey from world to world, searching for a way to defeat Lensa, they find themselves drawn to each other.
However, the curse is the real problem, and when the final showdown comes, it remains to be seen which is stronger-black magic or love.