Here’s a tip. If you only have one ebook for sale, write another one before you offer one for free. I’m always bemused by authors who offer free ebooks, without having anything else to sell. If I love an ebook, I want to buy other books the author has written. Therefore, when offering a loss leader, make sure that your book IS a loss leader – write at least two or three ebooks before you offer one for sale as a “taster.”
I've had more questions offering free ebooks as a promotional strategy.
If you want to do this, the above article will help.