Just a few days after the initial release, a second Chrome 66 based build has arrived in the Chrome OS Beta Channel. The update is build 66.0.3359.67 (Platform version: 10452.30.0) for those keeping score at home. This new build is available for the majority of devices according to the announcement and, more importantly, arrives for devices in the beta channel that can run Android apps.
I can confirm that my Acer Chromebook 15, which is in the beta channel, has received this update this morning. For the first beta build on Monday, it was not available to it.
As they often do, the Chromium team has released a partial change log for this update, which is mostly focused on stability and bug fixes. Reviewing the log, you will find that most of the efforts are addressing security and bug issues with the build as they prep it to go to the Stable channel.