As with many action/romance/comedy series (Shana, Index, Zero no Tsukaima, etc.), it's originally a Light Novel, which has currently been translated through Volume Two (with some change) by Baka-Tsuki. The manga and anime are both ongoing.
Incidentally, the anime is *probably* going to cover just the first two volumes (it's about the right length for thirteen or so episodes), and you'll basically spoil all the major events in the show for yourself... but you'll have the added advantage of *way* more information about what's going on, and can watch the anime as just the animated version so you can *see* the action. And the fanservice. The extremely copious fanservice.
Which is supposedly going to get even more so, judging by the author's comments. But you'll also see the things the anime's leaving out if you read the Light Novel, like Mil-tan. Who is hilariously awesome. You'll also see the original references, rather than the more generic ones they put into the anime.
So, I'd read the Light Novel, then go for the manga and anime as they're available.