I've been experimenting with two historical romances that have been on Kindle for a few months, using their final free download days before I take them out of KDP Select (which is exclusive) and expand by putting them on Smashwords.
With no promotion whatsoever, Mademoiselle at Arms reached a rank of 823 with total downloads of 1231. With a small amount of promotion (about 4 websites, Goodreads and Facebook) in two days The Conqueror's Dilemma is at a rank of 697 and the downloads look set to reach a little over 1000. There's still another day to go on this free period, but experience shows that if you stop promoting, everything slows down on the third day.
For good measure, Conqueror has made it to No. 18 in Amazon's Top 100 Free list for historical romance, and No. 97 in Romance. I didn't check Mademoiselle on these last time, but it too in the past has made it into these lists.
The first time I put the Conqueror on free, I did a massive amount of promotion, and it didn't break the 100 ranking, though it was close.
The big question here is how much have free downloads helped sales? The very first promotions caused a small spike which dwindled rapidly. The next ones did a tiny bit, but later ones did nothing.
Conclusions? Is it worth promoting free day? Well, it spreads the word. Putting it on free sites increases downloads and ranking a little. Also, Amazon do sometimes pick up the books and put them into recommendation emails. Both books have appeared on these a couple of times. There have also been spontaneous reviews for both books, which is a plus.
Facebook likes on dedicated book pages have not noticeably increased sales, and an ad on Facebook built likes (mostly from Italy) but did nothing else.
The next promotion game (apart from putting both books on Smashwords) will be to get the books listed on sites that don't require them to be free. I'll report back on that.
For the record, there's a trickle of sales on both books which now seems continuous. The trick is going to be to build these up. I suspect, from what others say, that more books is key.