The plot is slightly tricky, involving swoops back and forth in time as well as the identity muddle, but as usual with Diana Wynne Jones the underlying concept is solid as a rock. Somehow these children have accidentally revived an ancient and bloodthirsty presence, they have pledged their lives to her in jest, but now Monigan has come to claim her due, and she is deadly serious. The struggles of the 'ghost' to understand what is happening parallels the gradual comprehension of the reader, and the agonising (and sometimes very funny) battle to communicate the situation to her oblivious siblings, is tremendously satisfying. And the stakes are as high as they could be.
Diana Wynne Jones often treads a fine line between confusing and cleverly twisty, which means that her books reward re-reading. I loved The Time of the Ghost even more the second time around.