InDesign book has .INDD file extension
I noticed that in v. 15.1.2 an InDesign "book" file no longer has the .INDB extension, but is now an .INDD. If that is intentional, Adobe needs to update the User Guide at https://helpx.adobe.com/indesign/user-guide.html/indesign/using/creating-book-files.ug.html
The issue is not reproducible. InDesign Book has .indb extension.
S. Krupkowski commented
Update: Used Save As... and the Book file was saved with the proper .INDB extension.
Obviously a bug when it saved with .INDD extension. Screen shot attached.