I was expecting Shades of Empire to be free in the Kindle store today (see post below), but I was very surprised to find that in the US Kindle store only, my first book The Sixth Discipline is also now free on Kindle. TSD is not a Kindle Select book, since it is on other platforms; Amazon must be price-matching, as TSD is now free on many of those platforms. In fact I suspect its appearance as a freebie on the Nook Lovers site precipitated this action. Amazon has lowered the price of my books before but it never made one of them free except through a Kindle Select promotion.
Nook Lovers is a handy site if you have a Nook or use the Nook app; they list free and cheap Nook books.