For PbtA, I'd recommend Apocalypse World itself first, or Monster of the Week if that's more to your taste. (the writing style on AW is definitely polarizing, and the existence of the sex moves just bugs some folks).
For Fate Core, you can get all of the core .pdfs (Fate Core, Fate Accelerated, and teh System Toolkit) on a pay-what-you-want basis. When I couldn't get BW to work, I switched to Fate, and found it much easier to get going. There can be a bit of 'square peg round hole' stuff in Fate Core, and I humbly suggest the "Book of Hanz" for some guidance on some of the things that might work subtly different than they appear.
It, and the excellent Olaf vs. The Orcs sample situation both are available at http://www.evilhat.com/home/fate-core-downloads/