When you create a document (such as a book) and print it or save it on any hard-drive, thumb drive etc, you have simultaneously created a copyright to that document, owned by you. It will continue to be owned by you unless you transfer it to someone or some company. You can register the copyright with the US government if you choose, and this can make your copyright easier to enforce, but doing so does not create any rights, it just records them.
If you publish your document, unless you relinquish copyright ownership to the publisher or grant the publisher specific rights, you still own the copyright with the same rights you had previously. Publishing with CS does not transfer copyright ownership – it grants CS a non-exclusive right to publish the document. You can publish it elsewhere as well if you choose. You cannot, however, use a CS ISBN on versions published with someone else. If you use more than one publisher, they can each supply their own ISBN, or you can supply an ISBN and use it with all publishers (so long as the book is identical, otherwise you need a different ISBN for each version).