Seventeen-year-old Lilith Lane has a hunk of a boyfriend, a wicked tongue, and a talent for solving the mysteries that pop up in her small home town of Mirabalis. But when her little sister goes missing in the middle of the night, both Lilith’s detective skills and her recall of sophomore year physics are put to the ultimate test.
To save her sister, Lilith must race against the clock with the help of Dr. Hammer, a mad scientist who pushes Lilith off the edge – literally – of his reality-bending skyscraper. Lilith’s world is turned upside down by her sister’s disappearance, and a skyscraper that is also a portal to a parallel universe suddenly doesn’t seem that surreal. With Dr. Hammer’s help, Lilith must put her girl detective skills to the test and try to save her sister. "
This one is well liked: 4.4 stars on 25 reviews. This was originally released as a serial: even so it is only 162 pages (pretty short). Borrow for free if you have prime, though.