"Your first book was published in 2010, so how did you re-launch your Annika Brisby series so successfully?"
Well, you just asked a detail-oriented perfectionist a loaded question. I hope you've got your cozy pants on!
*Side note—I re-launched my series after I re-branded it. I learned about re-branding my covers and titles from Joanna Penn. I followed her instructions to the letter on her blog post for how to go about it. If you want to read about my experience with re-branding, check out my page Book Covers: An Odyssey.
As far as the mechanics of re-launching the Annika Brisby series go, I used Debbie Drum's Book Review Targeter to get the ball rolling. I found it by the happiest accident ever and the price was right, so I bought the software. It finds book reviewers on Amazon.com for you in a matter of seconds, and I built up a big list of reviewers to contact. Let me just say that this series is a blend of Urban Fantasy Romance & Paranormal Romance, so I had a BIG pool to fish from. It's also really competitive, so the cards were stacked against me. I started with 500-800 people but I added more constantly. I'll explain why later. I set The Flame and the Arrow (the first book in the series) to permafree. Using a Gmail template, I drafted an email with a picture of my fancy new book cover because a photo is worth a thousand words. No one wants to read a big long email from someone they've never met....let alone an email asking them for a huge favor. I told these potential reviewers exactly what the book is and what it is not, because I don't want less than the ideal reader downloading and reviewing it.
In my case, I explained that The Flame and the Arrow is about a wannabe rock star who meets an arrogant elven cad, and that there's angst and sex and adults using adult language in the story. Some people wrote me back and said, "WOW! This is exactly what I like to read! Thanks for thinking of me!!!" Some people wrote back and said, 'Yeah, I hate fantasy books with kissing so I'm not interested." I embraced those rejections because it meant I just saved myself from a 1-star review. Since The Flame and the Arrow is permafree at all retailers, I included a universal link to it in my pitch. Draft2Digital has that Universal link maker, and that's the bee's knees if you're distributed wide. Even though you're finding these reviewers on Amazon.com, a lot of them went on to post reviews at Barnes & Noble, Goodreads, etc. I also did this because Mark Coker says you have to make it as easy as possible for people to get your content, which means you have to eliminate every obstruction within your ability. Since the Universal link directs people to the free book at the retailer of their choice, that means every time they downloaded the book it counted as a sale, which helped my ranking. This was especially awesome for Amazon users because when they reviewed the book it showed up as a "Verified Purchase." And we all know how much Amazon loves those.
Now back to that big, fat list of reviewers. I imported them into a Google spreadsheet. I used the free Power Tools add-on to eliminate duplicates because the worst thing you can do is spam these poor reviewers! I used Yet Another Mail Merge (YAMM) to send out 50 queries at a time. You get 50 free sends every 24 hours with this mail merge program. There's an option where you can pay to send more or even all of your emails at once, but YOU MUST RESIST THAT URGE!!! I was trying to do this as cheaply as possible, so I was content to send out my 50 queries a day. Being cheap was about to pay off big-time.
It took some time to set up my list, draft my query email for reviewers, and organized my mail merge list. But once it was done it only took a few clicks to email 50 people all at once. Even though I could send emails every 24 hours, I didn't send them every single day. I was working a day job with limited computer access, so the days and times were somewhat sporadic. Still, I emailed 50 people a few times a week, and I did this consistently for 2-3 months. Those 50 query emails a day were sent 4-5 days a week, which brought a small but steady trickle of random shoppers to The Flame and the Arrow. As readers of my genre downloaded their free copy, the 'Also Boughts' for it began to populate with more and more relevant titles. After a while, Amazon's algorithms started doing more and more of the heavy lifting. I can't prove that with actual facts, but my results are real. I've been in the top 20 of my category since May 2016, and I've gained enough momentum that I don't query reviewers anymore.
There have been some great perks as a result of this process, such as finding people to add to my team of proofreaders and beta readers. They volunteered...I didn't ask. I've also found some die-hard superfans because of it.
If you're putting your books into KDP Select/Kindle Unlimited, I think this system would still work pretty well. All you'd have to do is tailor a few things here and there, such as focusing on finding people who have reviewed other KU books instead of ones that are distributed wide/traditionally published. That way your KU reads/free downloads would help boost you up in the KU algorithms. Also, you would have to adjust what links you send out in your email query. If your readers are in KU, just send them a link to the book you want them to review. And if they aren't in KU, you're still allowed to give out review copies as per Amazon's TOS. You aren't supposed to use services like Instafreebie because they're showcasing your book to an audience that's outside of Amazon. (If you're distributed wide, then Instafreebie is amazing for visibility, and if you're a paying member you can do this while growing your mail list!)
Some people will want to email their book directly from their computer as an attachment to each reviewer who asks for it, but I don't recommend that option. I can tell you from firsthand experience that it's a HUGE pain in the a$$ trying to figure out how to side load a book onto everyone's different devices. I pay Bookfunnel.com $20/year to deal with the tech support and it's been one of the best investments I've ever made as an indie author.
I do want to add the following disclaimer:
Your mileage may vary with this re-launch strategy depending on your genre, your book cover, your writing abilities, your people skills, whether or not Mars is in retrograde, etc. If you have questions, please look online before you email me! If a Google search can't answer your question, I'll try to help answer it, but you know that indie authors are busy people. I'm also not teaching this as a course, so I can't hold your hand through the process. That's why I gave you links to all the tools that I used.
Good luck with your re-launch!