There is no way for a project to load something out of another project entirely, never mind edit it. To do that safely it would have to load it—and then you might as well just load it and edit it yourself at that point. The closest thing to anything like that is the compile-only capability of inserting text from the disk into the compiled output on the fly, by using <$include:D:\path\to\common file.rtf>.
Something to consider, if you are writing a series of books that all depend upon common background information and research, is to put all of these books into one project. There are few downsides to having multiple books in one draft folder, and there are features to support working that way. If you’ve never done that before, try searching the forum a bit for discussions on how to do so. It’s a very popular technique and there has been plenty of tips and tricks shared over the years.
The other approach is to have a stand-alone research project that other projects reference against. There are tools to make referencing external items more efficient. Personally I would only go down that route for other reasons, like having so much research that routine backups take so long that they become inconvenient enough that you stop making them periodically throughout the day.