The following sources of information provide the best data available on American religion:
Pew Research Center
The National Congregations Study
The National Congregations Study (2012 data; report released in 2015) is a comprehensive study of what religious congregations in the U.S. actually do. Data collection began in 1998 and researchers are tracking congregational trends over time. In addition to the 2015 report, you can manipulate the data yourself with the tools they provide.
The Hartford Institute for Religion Research
The Hartford Institute for Religion Research has created an excellent portal page for religion research with links to numerous good studies, e.g., a database of U.S. megachurches (worshipping at least 2,000 persons per week) and studies on U.S. mosques.
The Public Religion Research Institute
Public Religion Research Institute conducts studies and provides e-news daily regarding the intersection of religion, culture, and politics. The CEO is Robert Jones, who authored The End of White Christian America.