It's no doubt that the KDP is a tremendous help for self-publishing authors. However, is select (being exclusive with Amazon) any benefit?
This option allows you to set your books free for 5 days out of 90. How well you promote those free days determines how high you get in the Amazon Kindle Free rankings. The higher you manage, the more downloads, the higher you go.
The problem? Most don't read the books they get for free. It's like a race to download free books and they sit on their kindle because A) they didn't pay for them and B) they weren't directly asked to review them, nor did they offer to review them.
Which means you've given out thousands of books and stand to get maybe five reviews, if you're lucky.
After research, it seems that while your ranking does go up during the free period, it does drop back down when returning to paid (sometimes this is immediate, other times it takes a bit - especially if you have active paid sales at that moment).
The good thing? You can always experiment. It's a 90 day commitment, but you are not locked in after that 90 days. You can opt to stay in, or elect to leave Select.
If you can manage to get your book lending -- you can average around $2 a book per lend. However, if you're book is $6.99 and not $1.99 or 99 cents, you are still loosing money on the lending option. So, I see this only benefiting those that sell their book for under $2. So, this is another option you need to weigh the pros and cons of.
Now there is the argument of trying to promote yourself - get yourself out there. Maybe a reader will take a chance on an author they haven't read before when they can borrow their book or give it a try for free. This is true - but, again - if they get it for free, it will likely sit on their kindle for an unknown amount of time before they read it - if they don't forget it.
In my opinion, a better option is (if you're a series writer) - release three or four books in your series, then price the first at 99 cents. You won't make much off it, but they're actually paid for it and are more likely to read it. Plus, they'll take a chance on a new author for a buck.
On the other hand of that, if you have three or four books out that aren't in a series, this could also benefit you. Price one at 99 cents to entice others to continue on to your others. The Prime free could work, but you're still (in my opinion) taking a risk. If it's a series, books 1 wouldn't be available anywhere else during a 90 day period.
Another consideration - many who are grabbing up the free books - only want free books. They won't actually purchase the next book - they'll wait for it to be free. And if it in't - they'll just move on to something else that is free.
Those willing to pay the 99 cents - i they enjoy book one and book two isn't priced like it's weight in gold, they'll pick it up.
I read an article, a time ago, where someone claimed they have 20K downloads over a three day period and was thrilled to have gotten 13 reviews out of it. What?
Alternately, you can use things such as THIS to assist with your free days. However, I don't see the point in paying to give way something (that's just me).
In the end, when making your decision, there are many factors to consider. Truth is, like with all tools for marketing, KDP Select simply works better for some than for others. Do you want all your treats in one bag?