Andy Watkins has this page with an amazing list of his ratings on hundreds of historical reference books. It’s worth checking before you take the plunge on a particular volume.
I picked up this book at a closeout sale and I’m really glad that I did. As a fan of the American Revolution period, I found it to be full of useful information. Duffy is a superior historian and writer, who manages to combine great detail with an easy to read style. Every aspect of warfare in this period is covered: the officers, common soldiers, sieges, set piece battles, and the home front. It’s a superior read.
Feedbooks has a list of some fifty free military and historical books.
Publisher TOR is offering a free ebook copy of Martha Wells’ All Systems Red science fiction mystery.
Rudi Geudens has a site that has images and descriptions of classic wargaming books. I’ve got some of those — or had some of those — and its a neat trip down memory lane.