Building on the original BeyondCorp paper in 2014, this paper is slightly more detailed and benefits from two more years of migration-fueled wisdom. That said, the big ideas have remained fairly consistent and there is not much new relative to the BeyondCorp paper itself (although that was a fantastic paper, and if you haven't read it, this is an equally good starting point):
The goal of Google’s BeyondCorp initiative is to improve our security with regard to how employees and devices access internal applications. Unlike the conventional perimeter security model, BeyondCorp doesn’t gate access to services and tools based on a user’s physical location or the originating network; instead, access policies are based on information about a device, its state, and its associated user. BeyondCorp considers both internal networks and external networks to be completely untrusted, and gates access to applications by dynamically asserting and enforcing levels, or “tiers,” of access.
As is often the case reading Google papers, my biggest take away thought here is wondering when we'll start to see reusable, pluggable open source versions of the techniques described within.