WorldCat is another form of permanent marketing. Most of you have probably never heard of WorldCat but it is a world wide book catalog where member libraries create entries for books in their collection they are willing to loan. These must be physical copies.
How do you get into WorldCat? Well, your first entry will be part of your self-publishing duties providing 2 copies to the Library of Congress. If you didn’t do that you had best take care of it. You can generally find the forms when researching filing for a copyright. Been a while. I remember you send 2 copies but it might be one for the Copyright Office and one for the Library of Congress. This is required of every publisher even self-publishers.
Your second entry into WorldCat comes from donating one or more copies of your book to a nearby university or college which is a member. Find the Web site for their library and you can usually find instructions on donations.
Another plus about having your work permanently listed on WorldCat is that Barnes & Noble as well as Better World Books seem to have automatic sales links. At least they do for technical books. Audiobook formats of novels even get listed.
It takes you a few hours of time and a bit of postage to fill out the forms and ship your books to a few libraries. Once done you have a permanent marketing source which may even generate some reviews. You certainly have a higher probability of getting reviews via this channel that a free book giveaway on GoodReads or LibraryThing these days.