I have a book to which I added two long appendices. On reflection it would make more sense for these to be in a separate book/project. How would I accomplish that? Can I have two projects on screen at the same time and just drag to two sections into the new project? I can see how I split an item at the point of the cursor but how do I split a project? Thanks
You can definitely do this. To have more than one project on your screen at once, simply load more than one at once. 
That’s about as easy as it gets. In your particular case, since you want to essentially fork your existing project into two pieces, I would use the File/Back Up/Back Up To... command to create an exact copy of your project in a new location (for this case, leave the zip option disabled). Now double-click on this backup project you created, and remove all of the material from it that wouldn’t pertain to the new book.
Now that you have both of these projects open, they can send files to each other very easily, as needed, with simple drag and drop between binders. So long as they both share the same labels and status, you’ll even get a 100% transfer of all meta-data as well.
Yes, I had thought of just duplicating the file in my finder and then renaming it and then removing the bits I didn’t want. I just thought there might be a cool way of doing it will a single click or so. Thanks for your reply.