Every paperback, hardcover, or mass paperback requires an ISBN. Some get both the 10 numeral ISBN but now, all get the 13 numeral ISBN. (They also need to be clearly in the book, usually the copyright page).
What is an ISBN? International Standard Book Number. They started with 10 digits, but then there was concern that the numbers would "run out," so they added the 13-digits.
If you aren't familiar with ISBN, you can go to their website at https://www.isbn-international.org/.
Amazon uses their ASIN, so some books get both. Some books, like Kindle Books and Audible.com (audiobooks), don't require an ISBN (which doesn't mean you can't get one) but they do get assigned the ASIN (Amazon Standard Identification Number).
If you self-publish and use a "do it yourself" such as KDP or Lulu.com, you can get an ISBN for free. You can also buy "ISBN" numbers and then assign them to your books as they are published. ISBNs can only be used once (for the book title AND its format). (Hardcovers and paperbacks have separate ISBNs).
Copyright. If you put it in your book, it's copyrighted and protected. You can go a step further and contact https://copyright.gov/ to get your book copyrighted. Ideas and thoughts, and titles cannot be copyrighted.
Library of Congress. This is something I even just learned not that long ago. If you want your books to be part of the library database, so libraries can order them or access the book, it needs to have a Library of Congress Number. Also, which I also learned recently, you cannot request it for books that have already been published. If you didn't get this number, you can republish your book, but it would need a new ISBN number and the request must be made before you publish the book. Go to the website... https://www.loc.gov/ Create an account and request a number. They will issue you one, which you put in your book on the copyright page, along with the ISBNs. After the book is published, they want you to send them a printed copy of the book. The Library of Congress does NOT issue numbers to eBooks. Only printed books.
I hope this helps.
