When do I need to use a new ISBN number for my books?

I just think that it's a shame that the United States isn't as forward thinking as some other countries (such as Canada) which provide them for free.  Bowker's goal seems to be to keep small players priced out of the market, which is why there is such a deep discount on higher quantities of ISBN numbers. Any advice that they give you is going to be the advice that results in the maximum quantity of ISBN numbers used within the rules of the international standards.

If you change the cover, you do not need a new ISBN since the text did not change. If you change the title then you do need a new ISBN as that is considered a new work.  You only need to issue a new ISBN if you make substantial changes to the text of the book, which would result in a new edition; a common guideline would be changes of between 15-20% of the work. So, correcting typos is considered a reprint and would be the same ISBN number; adding a few chapters of new material and revising existing material would be a new edition and would need a new ISBN number.

For me personally, I would not issue a new ISBN number if the content of the book remains the same and you are only changing front or back matter, especially if it is marketing text and not something relevant to the work (such as an index or glossary.)  Generally speaking, it won't make any difference which version of the book you buy if you only change a list of other books that the person should purchase, so it should not need a new ISBN.  It's just a reprint with some fixes, basically.

For electronic editions, there are two schools of thought. Each e-book edition (MOBI, PDF, EPub, HTML, DOC, etc)  should technically receive it's own ISBN number because the supply chain would need to uniquely identify each edition in order to ensure that a reader's device is receiving a format that it can read. This would be similar to the need for different bindings to receive their own ISBN number in print (i.e. softcover vs hardcover.)  Some folks are of the mind that you should have a separate ISBN for electronic editions but that you can use the same ISBN for each electronic format and not worry about it.

The International ISBN Agency ruled in February of 2010 that it makes the most sense from a supply chain point of view for each format to have their own ISBN number, but that there's a need for a separate number for tracking individual releases of the same book across formats. For me personally, when I begin expanding into markets where I need an ISBN number, I'm going to play it safe and assign a new ISBN to each format, but my distribution strategy is still going to only require 2-3 ISBN numbers per book.

My suggestion would be to have an ISBN number for your paperback version, a separate ISBN for your hardcover version (if you create one), and one for your ePub version. You don't need an ISBN for Kindle as there are very few devices that read MOBI instead of ePub these days and Amazon is basically the only marketplace for them that I am likely to use, and so there is no need for a separate ISBN as Amazon provides you with an ASIN to uniquely identify the book.  Having an ISBN for the ePub edition, that book can then be distributed through the networks that require one.

Your copyright page should include the ISBN number for the edition of the book that the read is holding, but to simplify matters I would recommend including all relevant ISBNs that you assign to that book (denoting which edition receives which ISBN number) so that it would be one less piece of data to update and maintain between editions and avoids the error of including the wrong ISBN and not having the correct one listed.