I don't mean real books, they don't normally get given away, I mean e-books.
It is now a common thing to see them advertised on Facebook and Twitter.
There are two main reasons for giving away a book.
1. To stir peoples interest in a new author's work.
2. To encourage interest in an established author's work.
The first one is the one I don't understand. The author spends ages writing a story and having it edited and proofread (no point in putting out your first work and it being let down on poor grammar). Then you give it away? Why would you do that? OK you might get some boost in FB activity and even a following but you have given away something you should value and ask others to see a value in it. So 500 people downloaded your free novel and have now got 650KB less space on their Kindle and it sits with the other 50 they have downloaded this month. 20 friends have now read it (the 20 guaranteed to buy it) and 5 others. You have 6 reviews on Amazon.
Was it worth it? For most authors I doubt it.
Lets look at case 2. The author is established and has 5 full novels available on Amazon priced at $5 and sells 25 per month with excellent feedback and reviews. The author now decides to stimulate interest in his/her work by giving away their best book. They get 500 downloads with 10 people reading the book. (More people read it because they feel it's a bargain). The readers love the work (you knew they would if they read it) and 8 buy the next book. This marketing takes into account the fact your friends have all bought the book and you are now selling to people you hardly know. Was it worth it? Yes because it got new interest in your work and possible sales of your other 4 books.
Another use for FREE books is as a 'loss leader' where you write short stories to promote yourself and get interest in other books sold at a reasonable price.
My advice is if it's worth reading then it's worth paying for. If it's FREE then mostly it's either bad or the author is daft/rich.
Sorry to all the authors that have promoted themselves with FREE books and if I am wrong then please tell me. Your story might sell your book better than giving it away.