After nearly five years of writing the Art Marketing Action newsletter, I finally (!) applied for an ISSN from the Library of Congress (LOC). This number is different from the ISBNs on my e-books. It's strictly reserved for serial publications and registers your newsletter with the LOC.
I urge artists who are putting out meaty newsletters with great content to do this. It's free of charge. Don't wait like I did. It's far too easy!