I would reccomend the Daniel Leary Series from David Drake. They're mostly available free from the publisher Baen. Here's the first couple to take a look at:
With the Lightnings
http://www.webscription.net/p-469-wi...ightnings.aspx
Lt. Leary, Commanding
http://www.webscription.net/p-258-lt...ommanding.aspx
And really, just check out what Baen offers for free on the webscription site.
www.webscription.net These are great books by recognized sci-fi and and Fantasy authors, and are actually for sale on some ebook sites. Baen just takes an open minded approach and figures if they can lure you in with their older titles for free, you'll buy their newer titles as they come out. In other words, don't let the price effect your view of the quality.