Miss Pym Disposes by Josephine Tey

Josephine Tey's Miss Pym Disposes is not boring, though if you stopped reading it halfway through, you might think that it is. 

Well-behaved, middle-aged Lucy Pym has become a minor celebrity after publishing a book of pop psychology:

"She read her first book on psychology out of curiosity, because it seemed to her an interesting sort of thing; and she read all the rest to see if they were just as silly. By the time she had read thirty-seven books on the subject, she had evolved ideas of her own on psychology; at variance, of course, with all thirty-seven volumes read to date. In fact, the thirty-seven volumes seemed to her so idiotic and made her so angry that she sat down there and then and wrote reams of refutal. Since one cannot talk about psychology in anything but jargon, there being no English for most of it, the reams of refusal read very learnedly indeed."

An old school friend who once stuck up for a bullied Lucy ("Lucy had gone home and enjoyed jam roll-poly instead of throwing herself in the river") is now the head of a women's physical education college (shades of a down-market Gaudy Night). She asks Lucy to come and give a talk about her book, and Lucy prolongs her stay, wrapped up in the world of the school, its rigor and rituals, and the personalities of the teachers and students as they prepare for graduation. Then, as the pages dwindle and the reader, perhaps baffled by the fact that this book has been billed as a murder mystery, is lulled, something shocking happens. Lucy's psychological insight is put to the test, and here is where Tey becomes a slight-of-hand artist, pulling off a satisfying, gasp-eliciting finale in a scant few pages, deftly turning the story from one thing into another. A mystery unlike any other, really, and a sly commentary on self-appointed experts. You gotta stick with this one til the end; it's not what you think.