Over the past three years I’ve seen the pendulum swing back and forth. Many say that gifting (or giving away) books helps sales. Others are adamantly against pricing their books for free even for a short time.
I’ve read accounts where free books have increased sales substantially on the free book involved (after the free period) and for other books by the author, particularly those in a series. I’ve also read the other side too: no increase at all, just crickets in the dark night.
Personally, I think sales are influenced by many things, including free offers. But the free offers must coincide with other things, like the time of year, the genre, other books offered by the same author, the alignment of the stars and moon and…many things that are too complicated to list here.