There are probably more free books for the Kindle than you could read in a lifetime. Searching through old books, now in the public domain, yields plenty of titles of interest and that approach accounts for the majority of items in my electronic library.
However, I have recently discovered another search string that yields a much shorter list of free, recently published books. If you search the Amazon Kindle store for -domain and sort the results by ascending order of price, it picks out those ones. As with the -public -breakthrough search I noted at the start of October, I do not understand exactly why it works but it provides a shorter list than that one by excluding the list of classics (which have already trawled).