Amazon has a feature for authors who publish directly to the retailer called KDP Select. Basically, you must make your work exclusive to Amazon and in return your work gets promoted to increase downloads.
But is it worth it?
Disclaimer: I did not opt-in to the scheme for Bane of Souls (you can buy it on iTunes and elsewhere), but may do so in the future for other work.
To answer that the situation regarding the eBook market needs to be summarised. There are many retailers, such as iTunes (Apple), Kobo (WHSmith), Diesel, Barnes & Noble and so on. If you opt into KDP Select you are definitely restricting the potential customer base you have, because your work will be available only for those with Kindles who buy through Amazon.
The mitigating factor is that Amazon remains the 800lb gorilla of book sales. If you did have to limit yourself to one eBook retailer it's a no-brainer that you'd go for Amazon. Roughly 98-99% of Bane of Souls sales have been through the Amazon sites. In short, if I *had* opted into KDP Select it would've made a small difference through lost non-Amazon sales but would've added promotion which *might have* led to a significant increase (or at least balanced the lost sales).
Some people I've heard of have gained a few hundred downloads, others thousands, and there is the possibility of getting some great reviews which will help the book sell later on. When you release a second book, if prospective buyers can see you've got a highly rated first novel it'll only shorten the odds of them giving your work a shot.
It's also important to note that KDP Select auto-reaffirms, so you have to actively cancel it.
I'm not going to opt into KDP Select for Journey to Altmortis (partly because I want to run a voucher scheme on Smashwords to temporarily cut the price, and don't know if I can do that with Amazon), but I do think it could be a clever idea for a short story. That way, there's promotion but it's of work that takes a few weeks/months rather than a year or more.
So, for me, it's not a clear yes or no, (which is unhelpful but true).
I might consider writing a short story (or a bundle of two), such as Horst's first lessons with spells or the story of how Pierre became Pretty, but don't think I'll be putting a full length novel onto an exclusive programme.