The Phantom of the East Tower - The Dinswood Chronicles, #3

Emma's freshman year has finally begun but not in the way she had hoped. Not long after arriving, Emma learns some shocking news: Doug has a new girlfriend. Although heartbroken, that's not all on Emma's mind. One night while getting some fresh air, Emma glimpses a mysterious phantom on the castle's east tower. Spurred on by other strange encounters, Emma launches into an investigation to find out if what she saw was real and, if so, what it could mean.
What she discovers is unlike anything she expected and sparks a secret adventure through the academy's underground caverns.  When an unforeseen danger threatens to undo all that she's accomplished, Emma must gather her courage and do the right thing. But what will it cost? And will she and Doug ever be the same again?