I currently have two books, one and two in a series, in KDP Select. When I published the first in the series late last year I was able to get it up to about 1,110 in the paid rankings on Amazon and then it slowly fell to about 400,000 in the rankings over the next few months as I worked to get the next in series out. Once I published the second book I put the first at .99 and did a countdown deal in March and was able to get it up to 1000 in the paid rankings. The second book got as high as 16k on paid ranking by riding the first books coattails. These are great rankings for me and since then the two books have been able to stay between 20k and 120k or so in the rankings (mostly through KU downloads, I think, with about one book sold every other day for each book) and I would like to keep them in this ranking range for as long as possible.
My question is: should I do another countdown deal for the first book or do a big free days promo? Which do you think would work better and have a longer effect on the books' rankings?
Another question: I'm liking how my also-boughts look on the books and I don't want to mess them up either. Would a free promo negatively effect this?