Sword Coast Adventurer's Guide. Back in March WotC released (for free) a similar book: Elemental Evil's Player's Companion and I was already really impressed with what it had to offer.
To me it's seemed like the entire system was designed with these kinds of books in mind. The fact that every class has dedicated sub classes means they don't have to reinvent mechanics from scratch or worse release something that claims to be new and it jsut a resin of what's alread out there. The same holds true with sub races as it allows for boos to exists that don't need to reprint information a player already knows. For example, all Elves get a bonus to Dex, but only High Elves get a cantrip because of their magical nature. Foresight like this lets designers play with ideas and minor mechanics without reinventing the wheel.
Sadly Sword Coast Adventurer's Guide isn't free as the last offering was, but I understand it is a business that needs to be run. From what I can tell there will be a lot to offer for the price tag attack ($35 not exactly cheap, but for WotC not a bad price). Check out what exactly is being offered HERE in Wizard's latest article on the topic.