The book I want to add has the isbn that was entered for this other book.  What can I do?

The book with the wrong isbn http://www.paperbackswap.com/Promises-Keep-Silhouette-Carolyn-Thornton/book/0373486391/#page-1

Link to Amazon showing this book with different isbn https://www.amazon.com/Promises-Keep-Carolyn-Thornton/dp/0373537026

This is the book I have that has the isbn that was used for the other book https://www.goodreads.com/book/show/15801525-harbinger-of-healing

You need to post your book without the ISBN. Hope the help center instructions below help.

When do you need to Post a book using the Post Books Without an ISBN feature?

• When your book has no ISBN printed on it
• When your book has an ISBN printed on it, but the ISBN is unknown to the PBS database (has not yet been added by regular updates)
• When your book has an ISBN printed on it, but when you type it in on the Post Books page, that ISBN "brings up" a listing preview showing another booktype (or another book entirely!) 
  • Such as the paperback if you have the hardcover, and vice versa
  • Such as Large Print when you have the regular-font version, and vice versa
  • This happens when the publisher assigns the same ISBN to multiple booktypes.

If your book does not match the listing information (title, author, booktype) in the database for the ISBN on the book, it must be Posted without an ISBN

To post your book without an ISBN:

• First, make sure to look over your book really well
  • Sometimes ISBN numbers can be found in unexpected places like the spine
  • The back of the book and in the publishing information in the front of the book are the most common places to find an ISBN.
• If your book has no ISBN (or the ISBN you have doesn't work in the system):
  • Go to the Post Books page
    • From My Account: click the Post Books tab
    • From anywhere on the site: place your cursor over My Account on the toolbar at the top of the site and click Post Books in the menu that drops down.
    • From your Bookshelf, click the link "Post Books" at the top of the page, in the yellow bar.
  • On the Post Books page:
    • Click the "Post a Book without an ISBN" link at the top of the page, in the yellow bar.
    • Enter the information about the book in the form that comes up:
      • Mandatory information includes: title, author, book dimensions, Subject (genre), booktype
      • Optional information includes: Large Print, Homeschool, Publisher, # pages, Back cover Description (synopsis of the book content), Book Cover image.
      • Your Book Cover Image needs to be less than 5 MB in size, and in .jpg format.

It's unfortunate that publishers re-use ISBNs, but it's really quite a common practice.  So it isn't actually a 'wrong' ISBN for either book, it's just that the ISBN was used in 1984 (Promises to Keep), and then again in 2012 (Harbinger of Healing).   Creating a non-ISBN listing with a unique listing number is the PBS solution for the problem.  It has some drawbacks, but it allows us to post those exceptions and trade within the PBS system.

Oh, thank you for that explanation.  That helps a lot.  I appreciate you taking the time to explain that.