It's a mini version of the rulebook with all the rules and none of the fluff. It's the most practical version to get if all you care about is the rules. You can buy a copy on eBay.
I would personally steer clear of the digital versions, they're difficult to use as a reference.
Is this a bad time to try and buy the rules with rumors of 6.5 or 7th ed coming out soon?
I doubt it will invalidate the old rulebook, best guess is it's just going to be 6th edition with the supplements included.