Unfortunately, you can't find answers to problems like this in a book. The water has to be coming from a pipe that is leaking, so you're just going to have to look harder to find it. Is there a pipe under the floor where the wet spots occur? Since water doesn't run uphill, if there is a leak under the floor it should show even more water under the pipe that is leaking. Since water can follow floor joists for some distance before surfacing, the leak may not be directly under the wet spots in the carpet.