Author Bob Mayer has several series that have been published traditionally. (some published under pen name Rob Doherty.) Six books are available free from today through Feb 25 on Amazon. Looks like they would all qualify as Thrillers, some series with a SciFi angle, some Conspiracy, all with a military slant. (Not surprising, since his bio claims some sort of prior involvement with US Special Forces.)
It looks like he's offering up one from each series. Rather than link directly to Amazon pages, I'll provide the link to his blog that gives good detail:
I haven't read any of his work, but they do generally fit into my reading preferences, so it looks like a good time to check him out.
A search of past MR posts shows that Mayer had replied to a post about an earlier giveaway, with some interesting details about the benefits he saw from ebook release of back catalog items.
I also found a really interesting blog post about what he sees as the benefits of ebook vs. paper book, and indie vs traditional publishing. (Spoiler alert-- he's going indie).