It should absolutely go without saying that you should be registering your work for a copyright as soon as it is finished. Ideally, before it’s even published. Now you have more reason than ever to do so. Read more about the ruling here. The gist of it is, you need the actual copyright registration in order to bring a suit for copyright infringement. The application itself is no longer enough.
If you’re confused by how things work, it’s like this: When you finish a creative work, you are the copyright holder by default. However, the status offers you no legal protection against infringement unless you register the work with the U.S. Copyright Office. Having your work registered gives you the ability to say, “This is my work, and I can prove it. If you try to steal it, I can use this proof to sue you for damages.”
The U.S. Copyright Office has an option to preregister your work, which is like a place holder for registering a creative work before it’s finished. This also counts and may be something for you to consider. Remember: your risk of having your work stolen increases with each person you send it to for feedback. It doesn’t need to be malicious, either. Someone can have their computer hacked, or their USB (with your book on it) stolen.
Bottom line: Always protect your work. You’ve worked too hard and too long to risk it all at the finish line.