A bunch of people have asked me through email and in the Writers' Cafe if it was a good idea to promote other books in a series, besides the first one and do I do it myself.
The answer is, "Why the heck not?" Sure, you want to do big promos on the first book, but why not promote the others, as well? Big or small, promoting will increase readers and revenue. For instance, if you promote book 3 in a series, it will sell well and many people will also buy book 1, even if it's at full price.
Consider my last promotion, a month ago. It was the most recent book in the series and it sold 2340 Kindle copies in one day. That's a good 2300 more than it was selling before the promo, bringing in over $1600 of additional revenue, at a cost of $780 for the ads. More importantly, book 1 in the series sold 228 on that same day.
I have another promo coming up in just eight days, with Fallen Pride, the fourth book in the series. This time, I have two other books that will be renewing in KDP Select and I hadn't used my promo days through Select on them yet. I'm talking about the second and third books in the series. Also, the first three books won't be eligible for a big promotion with BookBub for several more months. So, I've scheduled a short Kindle Countdown Deal for all four books, to run at the same time in the US and UK
Fallen Pride (book 4) will be in BookBub on 5/28, and I placed a few big ads ahead of it and a few other ads on the other books. The Countdown Deals all start at 9pm Eastern on 5/26, with ads spaced throughout the day on 5/27, BookBub on 5/28, and the CD ending at 9pm on the 29th for the first four books in the Jesse McDermitt Caribbean Adventure Series. Books 5 and 6 will remain at regular price.
Several hours before the CDs start, I'll update the book description of all four, inserting an opening line in bold type, "The first four books in the series are discounted for three days!"
My prediction is that Fallen Pride, being in the BookBub ad, will sell somewhere around 1500-2500 copies, while the first three books will sell around 200-300 each, for a total of 2100-3400 total sales of books 1-4 combined, for the one day of 5/28. Fallen Pride will be propelled into the top 100 at least, while the first three will rise to only about #600 or so.