How to open or close all documents in a book at once?

Assuming your book is active, this will close them. Can be reversed engineered to open them too.  

try {
 var b = app.activeBook;
} catch(e) { exit(); }
 var bcs = b.bookContents;
var i = bcs.length;
while(i--) {
   try { bcs[i].close(SaveOptions.YES); } catch(e) {}
}

There is a way to open multiple documents via the book panel, but it's a bit iffy.

Select the first document in your book panel, then hold Shift and click on the last document, this way all documents should be selected. Now double click (without holding Shift) on any of the selected documents and all should open. At least this works on MacOS.

Closing all documents can be done by Command + Option + W on Mac, this is a system wide shortcut to close all windows of the active program. I don't know if there's a windows equivalent for that.