Don't do free unless it is in exchange for signing up on your mailing list.
What percentage of people who download a free book will later purchase another book from that author? If I recall, the vast majority of people that download a free book do not become buyers. They are just looking for free books to read.
Studies have shown that, contrary to what you might think, people value something they paid for more than they got for free. The common "wisdom" is that people are more likely to complain about something they paid for if it disappointed them in any way. But, while there are people that do that (as well as people that will complain about free stuff), the more common reaction is for people to justify their expenditure by highlighting what they liked. Plus, there is the social proof in that if you buy a book and say you liked it and several other people buy the book and say they liked it too, well, that further proves you have good taste, right? And if people are more likely to praise something they bought than something they got for free, that's a series of wins for the you as the author in the case of a book.
Another way of looking at it is to pretend you're an illegal drug dealer. Yes, some dealers will give someone a free sample because there are certain drugs that are addictive from the first use. So, they know you will be back for more and that you'll pay whatever for your next fix. Cha-ching!
Okay, books are not as addictive as drugs. You can pretend you have the most addictive book series ever--and maybe you do--but books are probably never going to be as addictive as drugs unless they're paperbacks and the pages are dusted with drugs and you get a contact high every time you read a book.
But, never mind all that. Let's go back to you as an illegal drug dealer. Now, the reason you gave a free sample was because you knew they would be back for more. Plus, again, what you're doing is illegal so where else is the person you gave a freebie to going to go? Someone else might be an undercover cop and they want their fix not to go to jail. So, more likely than not, they are going to come back to you. Again, cha-ching!
Now, let's say drugs are legalized. Yay for you, you think. More customers! But, also, more competition. Now, there's a dealer on every street corner. But, you continue giving free samples. Your competitors do the same. However, now the addict has more options. He doesn't have to come back to you to buy drugs. He can get his fix by getting a freebie from one of your competitors. And, since there are so many competitors, he can keep going from one to another, never paying for any drugs, but still getting his fix.
But your drugs are better? Safer? More effective? Yeah, but, you know, they're free from the other guy.
Again, we are talking about highly addictive drugs and if free isn't going to work to bring customers back to you for that, do you really expect it to be an effective strategy for getting people to buy your books which are far less addictive?
Maybe it worked years ago when there was far less competition but now?
