Along with iOS 11.3 developer beta 2, Apple released macOS 10.13.4 developer beta 2. The beta 2 renames iBooks to Books, bringing the app inline with its iOS counterpart.
macOS 10.13.4 also includes an alert for 32-bit apps, support for Messages in the Cloud, and a privacy icon description similar to the one found in iOS 11.3.
You’ll need an Apple developer account to download the beta, or wait for the public beta to come out. The betas, just like official public releases, are free downloads.