American Society of Composers,
Authors and Publishers (ASCAP)

National Music Publishers' Association (NMPA)
U.S. Copyright Office Home Page
10 Big Myths about copyright explained
Copyright Resource Page
The Copyright Website

Something to Keep in Mind:
In most countries, even if there is no copyright notice, the composer retains the copyright. This means that you cannot assume something is not copyrighted just because you cannot find a copyright notice.