Others See Us — William Sleator

After Jared accidentally plunges into a polluted swamp, he gains the ability to read minds . . . and discovers dangerous secrets about his family

The first thing Jared does every summer when he and his family arrive at their cottage is hop on his bike and cruise along the back roads. Only this year he's grown too big for the bike. When the brakes suddenly give out at the bottom of the hill where the road makes a sharp turn, Jared plunges into an industrial swamp oozing with toxic waste.
 
After the accident, Jared feels OK -- except for a headache. But then he starts hearing strange things: people's private thoughts, which are the total opposite of what they actually say. Next, Jared's journal is stolen. Luckily, he can just read the mind of the person who stole it. He can also use his new power to track down the culprit in a recent series of ATM robberies and neighborhood break-ins. But along with solving mysteries, Jared uncovers shocking family secrets, the identity of someone else who has the same paranormal gift as him, and the truth about the girl he loves.