I've been looking through drivethrurpg.com for quite some time now. I've seen Vampire: The Requiem 2nd edition, The God Machine Chronicle, etc. But it's difficult to determine which book is the definitive New World of Darkness Core 2nd Edition.
What you're looking for is the Chronicles of Darkness core rulebook, released in advance form in mid-December 2015 to commemorate the rebranding of the nWoD lines. You can acquire it at DriveThruRPG here.
Currently is the only book out that is solely the new rules without any splat is: World of Darkness: The God-Machine Chronicle which is just an upgrade / translation guide towards the nwod 2nd edition.
As Nyoze has pointed out the "true" 2nd edition nwod book is currently still unreleased but will come out. So far there has been no info if it will contain exactly the same infos as the god machine chronicles or if some things will get tweaked inside of it.
Additionally EVERY book released so far for the god machine chronicles / 2nd edition nwod have had most parts of the god machine chronicle upgrade rules included in them. If this will stay so AFTER the 2nd edition book has been released is to be seen.
So current status is: 1st edition world of darkness rulebook + god machine chronicles upgrade are the current rulebook. Then soon to follow is the 2nd edition rulebook itself which will replace that.
It hasn't been released yet
While you can use the updated rules contained within Vampire: The Requiem 2e, and other updated 2e products, the Second Edition Core nWoD rulebook is currently slated for release in the Northern Hemisphere's Summer of 2015.
Check out the publisher's website at The Onyx Path for more information.