I reduced one series to 99 cents for all thirteen books, and made sure that each week a couple of books were available for Kindle Unlimited. Not very strategic, and I didn't want to appear to be in any way taking advantage of people who were hurting.
I sent a few paperbacks to nursing homes, and then stopped because some didn't want to bring in outside items. I'm going to try that again now that it seems COVID-19 is not generally spread through contact with inanimate items.
My broad suggestion for sales during a time of crisis is the standard one -- consistency. I've expanded my number of tweets and continued to update my webpages. Here are a couple of other things I'm doing. I can't vouch for their effectiveness, but they make sense to me.
1) Reexamine your book categories by studying those of similar books that sell well, or books that address similar topics. For example, I looked at how Amazon categorized The Twain Does Meet and saw two categories addressed culinary mysteries. (The book does mention that Jolie is not a very good cook, but there are no recipes.)
Thinking maybe I could draw in readers looking for the subject matter rather than fiction, I decided to look at books on multiple births or new parents. I saw two categories I liked --#225 in Motherhood (Kindle Store) and #3913 in Family Life Fiction (Kindle Store). Using the contact link for Amazon (from within the book's page in my account), I asked that two categories be removed and these two added.
Amazon has a good system for this now -- they offer options for the topic you want Amazon to help you with, and on the one for the product page (where readers buy your book), the first option is categories. Only time will tell if it makes a difference. I'm doing the same thing for many books. Labor intensive, but it makes sense.
One hint. You want the category that says "Kindle Store" for your ebooks and the once called "Books" for your paperbacks.
2) Audiobook sales vary greatly for my books. A series gets a boost when a new book comes online, but that's not usually a monthly thing. I sell mine through ACX, which places digital books on Audible, Amazon, and itunes. There are a lot more options today, but this has worked for me. ACX recently made changes to the promo code system. Authors use these to give free copies to reviewers or others.
I didn't realize I could get more codes, or codes for some early books for which I had never requested any. Go to your sales dashboard on ACX, and just above the list of books sold you'll see a link for "Promo Codes." When you click that, the list of books eligible for codes appears. Click on each one and request the codes -- for the U.S. or U.K, or both. You get the codes almost immediately, and they are active after about an hour.
I sent my newsletter folks a list of books for which they could request codes, and received many requests. (I could offer eight books.) With the click of a button, the new system creates the note you can send to requesters telling them the code they need and how to request the book.
The new system also helps you keep track of those you give out. ACX will now tell you if the codes are used, and you'll know to whom you gave them. A couple of weeks after the recipient uses the code, you could drop them an email to see if they enjoyed the book. That could encourage a review.
3) Back to the one word that makes the most difference -- consistency. I send a batch of tweets every, single day. Beyond that, I mix up ad placements, mentions on relevant Facebook groups, blog posts, and regular newsletters. (I continually gather new subscribers.)
Good luck, and feel free to note your ideas in the comments section.
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For more information on Elaine's 30+ books, go to https://www.elaineorr.com or subscribe to her newsletter.