Not many of us have the luxury of
buying our own home having a dedicated home office/study AND a guest bedroom. In our three bedder townhouse, we have to share those spaces in a small room. In our next place (or own home), I’d be considering this:
A short ‘wall’ or ‘divider’ made out of two bookcases joined together. On one side, they feature shelving for books and things. On the other side, they become the backing or the short wall against which your desk can butt up. If you make the bookcase a little taller, you can use the backing as a pinboard. Even hang a few hooks…
That way, none of the room loses its natural light (if positioned so) and you don’t end up with two cramped spaces.