I've just posted links to all of the Seven Years War books I've been able to find so far that are hosted on Google Books and the like. I'm sure somebody has compiled a better, more comprehensive list of public domain SYW books, but I haven't been able to find one yet so I've just stuck up what I've found(approx 40).
Mostly, the originals have been scanned in by university libraries meaning that the viewing quality of the books varies. I've only found time to read a couple so far (can't read books on a computer screen to save my life) but of the ones I've read I'd recommend the biography of Seydlitz. It's full of the funny little anecdotes that make this period a pleasure to study, and the quality of the scan is very good.
You can get to them by following the link in the top right of the blog or clicking here.