Unlatched — Steven Savile

Matt Fisher was a normal boy--until he found the Door. The Door that led to the House. The House whose Doors opened into places all over the world--and into worlds that had been, and would be, and even never were. But Matt wasn't the only one who'd found his way in. Now he had something the man named Twig wanted, something that could sow the seeds to everything's destruction. This first book in the spine-tingling Latchkeys series sets the stage for more adventures with Matt Fisher and the rest of the Latchkeys Kids!