There are several factors that influence the success of a giveaway as a tool to sell books. The first is what platform are you giving your book away on? If you post a book on your blog and allow folks to download it for free, that will help you get readers only if your blog already has a ton of followers; and if you post it in Mobi or epub format, you will only get readers who also know how to side-load a book to their ereader. Putting the book somewhere that people go to find books is a much better plan. Smashwords makes it really easy to make a book free, and Apple iBooks and Barnes & Noble Nook will price match that, so you get more bang for your non-buck than you would just from Smashwords. Sony used to price-match, but it doesn't look they they're still doing that. Generally speaking, the Kindle store will usually yield the best numbers, in terms of a giveaway, but even with a book being free in the Kindle store, you need to advertise it, especially because unlike Smashwords, Amazon doesn't make it easy. You can only make a Kindle book free if it's in the KDP Select program (which means it only be sold as an ebook through the Kindle store), and it can be free only five days out of 90.
So many people have taken this route that it is actually difficult to give away a book in huge numbers. You should advertise the book being free on your blog, on Twitter and Facebook, and anywhere else you can find. There are lots of site to do this on, but be aware that many of them either charge up front or ask for donations.
I have made three books free in the last year or so, at different times. I launched my two most recent books on KDP Select and made them both free on different days. I gave away several hundred copies of each book, and after it was free, I did find I got more reviews and more sales after the books came off being free. I didn't try to make my first three books free on Kindle via KDP Select because there were already out there on many platforms and it is often difficult to get smaller vendors to pull books back when you “unpublish” from Smashwords. I did make my first book free on Smashwords, and after iBooks and Nook stores showed it as free, Amazon price-matched it to zero (although only in the US Kindle store)*, and I have now given away well over 2,000 copies of it! The big advantage to me is that my first book The Sixth Discipline has a direct sequel called No Safe Haven. NSH has only one review on Amazon, and yet it is now my best selling book! The increase in sales to NSH from making TSD free is much greater than the increase to the other two books that were free for a time.
Even if you only have one book out there, making it free in the Kindle store helps by getting your book into many readers' buying history (“People who bought this item also bought . . .”). It can also increase your chance of getting reviews.
Things to keep in mind when doing a promotion:
- Not everyone who gets your book for free will read it.
- Not everyone who reads it will like it; it's always possible you could get a negative review from a free copy, especially if the book doesn't live up to (or match) the blurb.
- There are tons and tons of free books out there, so you need to get the word out!
- The market size of your book still applies, even when it's free; check out the top 100 free Kindle books to see the genres and authors that are doing best.