You can use the 'merge' function : File>File Special>Merge then select the Layout book option - this will import all the layouts and Masters from the selected project. Then delete what you don't need...
If you just want the one Master, open the Organizer, click the pop-up in the upper left corner, choose Browse Project... to open another file, then drag and drop the Master into the current project.
David
David Maudlin / Architect www.davidmaudlin.com Digital Architecture AC27 USA • iMac 27" 4.0GHz Quad-core i7 OSX11 | 24 gb ram • MacBook Pro M3 Pro | 36 gb ram OSX14