When you're writing a series of books -- either a fiction series or a set of non fiction volumes -- it's tempting to publish each book as soon as you finish creating it. After all, books that aren't for sale don't make you any money, right? This may be true, but when trying to get a series off the ground sometimes it's better to give up a little extra money now in exchange for a lot more money later.
It's all because of the way books sell online. 
Once a book is new on the virtual book shelves, it's got a certain natural life before it starts to get old and start slipping down the rankings. The lower the ranking, the fewer people who will see it and possibly buy it. The only thing that can extend that natural life is by adding something into the mix to keep reader interest high. A brand new book in your series, with a link to the older book inside the back of the book, is the ideal way to lift your first book back up. Add a third book into the mix and the rating of all three should rise and stay prominent longer.
How to Set Up a Long Promotion
Counterintuitive as it may seem, the best idea is for you to create and bank at least three books in the series. This method works especially well if it's a new series, or a series on a new pen name. Write your books as usual, have them edited and polished, then save them on your hard drive and in a thumb drive or in the cloud. Insurance never hurts. Have covers for the entire book series done at once, using the same designer, and make the covers look like they belong together. When you have at least three books finished, begin your publishing and marketing plan.
Starting the System
Schedule your books to drop three weeks apart, unless they're romance novels, in which case they should publish every two weeks. The rabid appetites of romance readers guarantees a quicker turnover in that genre. Once you have the first book published, add a link to the sales page to the back of the second and all subsequent books. Publish the next book so it hits before the first one drops in the rankings. Once that's published, update all your books' back matter. Continue with the third book in this same manner. The whole time you're publishing these books, be writing the next one to continue the series.
Concentrate almost all your book marketing on the first book in the series. The idea is to get your readers hooked so they buy the rest of your series. Once they see how many books you have high on the listings they're more likely to buy them, which will push their rankings up even higher.
Keep the Faith and May the Force be with You!
