Sorry everyone--figured it out. The setting IS in preferences, I had inadvertently changed "wide" to "narrow". Changed back, & voila! All is well again.
Please disregard! & thanks for being here...
So, book details window used to be a narrow pane on the right hand of the user interface. I quite liked it there, very handy & useful & easily glanced at. Unfortunately, it seems to have moved itself (or I have inadvertently done something to move it), & I cannot figure out how to move it back! Nor can I find anything in preferences, frequently asked questions, or the user manual which seems to address the problem.