The ISBN on your book is important and if you hope to sell your book, you will need an ISBN.
ebook
So maybe reading a book on a computer-like device – instead of feeling the smooth paper as you turn the pages – isn’t for you. But even if you don’t personally like reading eBooks, if you’re an author, you need to make sure you’re selling an eBook!
If you want your book to be carried at a library, you’ll need an LCCN.