Confusion, by Stefan Zweig

I was in the mood for a mystery story of some kind, and this short novel hit the spot.  In it, an elderly literature professor reminisces about his youth--about his upbringing as the son of a schoolmaster in a country town, the initially rocky start to his college career in Berlin, where, like so many students in our days too, he neglects his studies and falls into a dissipated lifestyle, and how he transferred to a university in a smaller town with fewer distractions and met the professor whose passion about Elizabethan literature inspired his own career.

There is a mystery about this professor, whose brilliant lectures about his subject on some days alternate with humdrum, ordinary teaching on others, and his lack of major publications.  Then there's his unlikely choice of wife, an athletic woman much younger than himself who shares none of his scholarly interests or habits and whose relationship to her husband seems strangely detached.  Add to that a habit of sudden, unexplained absences for days at a time, when even his wife has no idea where he is, and erratic behavior, when he alternates between obvious enjoyment of teaching and working on his book with his student and brusque dismissal, and you can see why the student is confused and becomes determined to figure out what's going on.