limone: emellaich is right on this one. Case in point:
I've actually found that you sell more, and have happier customers if you DON'T use any DRM at all. DRM only annoys and punishes the honest. It does nothing to stop the dishonest. If you are really that scared of someone stealing your stuff, you've got your eyes on the WRONG THING. Writing and publishing IS NOT nor ever should be about the money. It's about writing, doing what you love and sharing it with others either for free, or for purchase.
If you're in it for the money you're doing it for all the wrong reasons and it'll blow up in your face and be an epic failure for you. On top of that, you're likely really new to all this if you're asking questions like this, so here's my advice. It's not piracy, or theft, or anything like that which is the bane or curse of the writer. It's OBSCURITY. Yes, it's plain old obscurity, with people not knowing who you are. So here's my advice. Forget about the money, forget about the DRM, and forget about piracy. Focus on getting readers, making them happy, build a fan base, build readership, give value added extra content with your books, etc.
Once you get yourself a sizable fan base, THEN consider changing your business model to something else. But for now you want one that's as open and friendly to the end user as possible. IF that means free at the beginning, then so be it. That's actually how I built my initial fan base. I used to write short stories and give them away on my tech site for all the read and enjoy. Then when I moved to novels I did the same, giving away one chapter at a time. Now my fans are dedicated enough that when I release a book they're more than willing to pay whatever price I ask, and some even send me money without asking anything in return. Yes, I kid you not, they actually send me money out of the clear blue to "support the artist" as they say. That's the thing you should aspire to. It sure beats the more restrictive, controlling business model you're planning.
Well, that's my 2c. I hope this healthy bit of advice from a seasoned author helps.
PS: Since you're so new to book publishing (at least that's what I'm taking from your post), I would suggest you also read my guide to book signings and events. Even if you only do ebooks and don't do a mixture like I do (I still do print as I have a number of fans who demand it, thus I carry it) it'll still be handy as even in the digital ebook age there is still a place for doing book signings and events for authors.
Check my website for details.