(Microsoft officials said there is no new PC test build today due to an unspecified "blocking bug.")
Build 14977 adds the ability to read EPUB file-formatted ebooks in Microsoft Edge. It also includes a change in the rendering technology used for many types of Universal Windows Platform apps, as well as some other relatively minor tweaks around OAuth, notifications customizations, and more.
On the known issues front, Windows 10 phone users with 8GB of internal storage -- like the Lumia 550 and 650 -- will receive errors when attempting to update to this build. System related notification toasts don't work. And testers should avoid attempting to install new languages, keyboards, and speech packs with this build, Microsoft officials warned.