After being first released last week, a second Chrome 66 build is now available in the Chrome OS Stable Channel today. Build 66.0.3359.158 (Platform version: 10452.85.0) is available for most Chrome OS devices starting today and is the build that systems which are capable of running Android apps will be updated to run.
I can confirm that my Google Pixelbook was updated to build .158 earlier this morning when I used the chrome://help command to check for the update.
As for updates & changes, there are plenty in this update.
Here is a complete rundown of all of the new features or updates found in this new Chrome OS Stable Channel build:
- Adding new keyboard shortcut to move windows from display-to-display
 - New Chrome OS Keyboard Shortcut Helper
 - External Display Settings enhancements
 - Enable Video Recording in Chrome Camera App
 - Overview window animation improvements
 - Extending more devices support for Magic Tether
 - Picture in Picture Magnification
 - Ability to zoom up to 20x with Chrome OS magnifier
 - Ability to adjust full screen mag zoom level through pinch gesture
 - Screen sharing support for Android apps installed via the Google Play Store
 - Add Google Play into first login Opt-in window
 - Google Play GDPR support
 - Google Play Maximized Window support improvements
 - Automatically pass user credentials from Chrome OS login to network 802.1x authentication
 - Native printing support extended to Google Play applications
 - Add Sync notice during initial sign-in