Unless it is a performance issue, I think having more than one project open at a time is a decent way of working. You get more by that, such as being able to jot down notes that might be pertinent to the whole group of books using those notes. If it’s a screen space issue, consider how compact a Scrivener project window can really be (all the way down to a flat rectangle that only contains text and a menu within its borders (or other editor view content). All of the toolbars and sidebars can be turned off, making it no more than Notepad in appearance.
Save that as a Layout for when you’re using this project as a reference for other projects, and another that uses a more conventional setup so you can switch back easily (as it does take about half a dozen menu commands to get it that minimal). Here’s a screenshot of how that could look, on a laptop screen. For that screenshot I left the editor header & footer bar on, but those can be removed as well.