I used to be able to shift select multiple pages in a spread with the page tool and it would display the width of the whole spread in the width panel. Now it is blank. Did that feature go away?
Are you saying the Width field is empty? If so, that would indicate different widths are selected. Click on one page at a time and see if there are two or more different widths.
You used to be able to click on all pages within a spread like a book cover (back cover, spine, front cover) and it would calculate the width of the spread (all the pages combined regardless if they were different sizes as individual pages)
I've been teaching InDesign since 1999 (the first year it came out) and for years I've taught how to create multipage spreads as you describe. The Width field shows the width of the current page. As David Creamer said, if you are using the Page tool, and have multiple pages selected with different widths the Width field shows empty. If they are all the same width, it shows their page width. InDesign has worked this week since the Page tool was added. It has never shown the sum of page widths.
My typo above: It should read, "InDesign has worked this way since the Page tool was added...."
I don't recall the page tool working that way either.
If it helps, you can change your ruler unit origin preferences to spread and the top ruler will give you the width. Drop a guide on the right most page edge for an exact measurement.