A family of Microsoft word processing software products for creating web, email, and print documents.
Thanks for asking. The answer is no, there will not be another edition. You can see at https://www.microsoftpressstore.com/series/series_detail.aspx?st=99023 that there are no books in the Plain & Simple series beyond Office 2013.
The editor at Microsoft Press told me that the market for books that cover all the features of an Office program has shrunk below the point where they feel it's worth publishing them. Instead, they plan to focus on "vertical market" books, such as (maybe) Word for attorneys, or Excel for accountants, or whatever looks promising.
From an author's viewpoint, the monthly stream of new features in the Office 365 subscription editions of the Office programs would require constant updating of a Plain & Simple book. I certainly don't have that much time to devote to it.