I've started a promo for Historical series (any genre) on Bookfunnel. The requirement is that your books all are available for readers who subscribe to Kindle Unlimited. Full price, bargain price, perma free... are all fine, just as long as the books are in KU. The idea is to display the 1st, 2nd, 3rd in the series...displayed together. If you only have a 1st and 2nd, then display those two together. if you have a prequel you want to display, then you would add the 1st and 2nd to be displayed. This is not a "bargain" or "one time only" sales announcement promo. It is essentially an author newsletter swap. All participating authors promote the promotion. (For my part, I typically bring 400 to 800 views to the free books promotions on Bookfunnel. I have no idea how many I will be able bring to this type of promo) Best to put your books in the promo all at once so they will be side by side or close proximity to each other for full impact. The link is here...
https://dashboard.bookfunnel.com/bundles/board/om0zxki05h