A young adult story about a boy finding a kidnapper’s journal on the subway

It’s a small chapter book with a blue/light blue, mosaic image on the cover. A boy finds a journal on the subway, the setting was some kind of city. As he begins to read through it he realizes it includes a plan to kidnap someone which was going to take place soon. The boy is captivated by the idea that he could prevent something happening and begins to search for clues on where to find the journal’s author by reading through its pages. He keeps reading and gets closer and closer to finding the person. At one point he’s in a house and is sneaking into a basement of some sort assuming it’s where the kidnapped person could be held.

I remember being hooked on this book when I read it as a summer reading list book in middle school. This was around 2006-2009 in Texas.