If you're self-publishing your book, below is a checklist of items you should complete before publication.
Have an attorney complete a legal read of your manuscript for libel and right of privacy violations, and any intellectual property infringements.
Look into obtaining insurance for authors to protect yourself if anyone brings a lawsuit against you.
Draft a collaboration agreement with any co-authors to determine ownership, payment, responsibilities, and credit, among other things.
For anyone you have hired to work on your book (e.g., editors, cover designers, illustrators who are not co-authors, etc.), have an attorney draft a work-for-hire agreement that says those individuals do not own any copyright interest in your book and are not entitled to any proceeds.
Obtain an ISBN and LCN number.
Have an attorney review the self-publisher's contract, if any, to know your rights and obligations before publication.
If you're using Amazon to self-publish, you will likely need to prove ownership of your book. Investigate the paperwork you need to submit to prove you own the rights to your book.
Register the copyright in your book.