Set up book reviews and book readings. If you're professionally published, check with your publisher first to see who they've sent copies of your book to. This way, you can send copies out to a new pool of readers. Look into blogs that cover topics similar to your novel, but only schedule a few reviews at a time. Let the reviews dribble in. Doing so allows you have a steady stream of reviews over the next couple of months instead of getting an immediate spike of sales in the beginning that taper off after only two months. While the book reviews will be for your newest novel, the reviewer can make mention of past publications as well.
Write guest blogs. There are many blogs out there where the creators rely on guest entries to help build their site into more than it would be with just a single writer. Along with guest entries, you can also take interviews. Sprinkle in a fact or two about past works to build interest, but focus mainly on your most recent publication. Remember, it's your newest baby. Show it off, but don't forget about its siblings.
Set up book signings. When you have a book signing scheduled, make sure the store has copies of the newest installation in your series as well as those that came before it. Folks who come in to browse may see your books for the very first time and want to purchase a bundle. Even if they only purchase the first in the series, by seeing you there at a book signing, they know where to purchase books two and three later on.
Call one or more nearby newspapers or send them a press release about your newest book. Tell them you're trying to build attention for your work and want to see what they can do to help. They may be interested in promoting a local author and could ask for a interview for the paper, or they may run a review or mention of your book in the local interest section.
Promote your personal blog or website. Having a website allows you to place images of your titles on the front page as well as links to pages on Amazon, Barnes and Noble and other online bookstores where your book may be available. This gives your readers an option to make their purchase while reading over each of the titles.