Indesign doesn't really have the capacity to gather elements and create multiple versions of the same book. This is what Adobe Framemaker does. It takes pieces and combines them into a working manual/book. Framemaker also allows you to use the pieces in several Framemaker documents.
If you aren't interested in Frammaker, or don't have this need on a regular basis. Then Lauren's comment about simply duplicating the original file and using that, is appropriate. Indesign locks pieces into its "books". If you need the same piece in multiple books, you need to duplicate the piece. This can create problems when edits are needed.
Perhaps a better option using Indesign is to create a new document, the same size and page count as
filename2.indd. Lets call this new document
NEW.indd. Then use File > Place to place all the pages of
filename2.indd into the
NEW.indd document. What this does is merely create uneditable representations of the indesign pages. So... if you subsequently update or edit
filename2.indd you can then open
NEW.indd and update the links. This is generally much easier than trying to apply the same edits to multiple documents.