If you seek to sell your book or even if you are considering commercial distribution, you’ll need an ISBN. If you intend only a limited distribution to family and a few friends, you probably won’t need to get one.
The US Copyright Office defines copyright as “a form of protection grounded in the U.S. Constitution… (it’s) a form of intellectual property law which protects original forms of authorship.” Work is under copyright at the moment it is created. Registration of a copyright is voluntary, but you must register if you bring a lawsuit for copyright infringement. The Copyright Office advises, “Many choose to register their works because they wish to have the facts of their copyright on the public record and have a certification of public record.” Registration within five years of publication is considered prima facie evidence in a court. The basic fee to register a copyright online is $35.
As with an ISBN your intentions for your book will dictate what you should do. If you work with a commercial publisher or an author service company like iUniverse, Create Space, Author House or XLibris make sure you understand who controls the rights (meaning copyrights) to the book.