For a while now, I've been using FreeFeedRead to keep an eye out for good free Kindle books - since Amazon do clients for PC, Mac, iOs and Android in addition to the Kindle itself, it's a good source of reading matter.
If anyone else is interested, go to http://uk.freereadfeed.com/ (or your equivalent country!) and plug in some search terms. I find the best approach is to select 'Pick From A List' for 'Genres', pick 'History' and also select 'Also in P-Back', as I find that can weed out a lot of the dross. (Having said that, a couple of my last few acquisitions were Kindle-only :D)