Snapdragon Alley, by Tom Lichtenberg
Ten year old friends Alex and Sapphire discover something strange on the city bus map, a street that existed for only one year. As they set out to solve the mystery, they encounter the possibility of another world, another dimension perhaps, in a vacant lot, but they are not the only ones on the trail. Who will discover the truth, and who will pay the price? Book One of the Dragon City series.