The back cover (and probably the front too, if you know how to read the code) says that 古書店アゼリアの死体 （Koshoten azeria no shitai, The Body in Azalea Old Books, 2000) by WAKATAKE Nanami (若竹七海, born 1963) is a コージー・ミステリー, or 'cozy mystery'. There are quite a few detective stories that are 'comfort reading' for me; but I'm not quite sure what the genre rules of cozy mysteries are. I'm guessing that typical features would be: having more appeal to women than men; some likeable characters in major roles; a romance somewhere in the plot; treating something from daily life (like handicrafts) as a subject of specialised knowledge; a lot of humour, but not too wild; restraint in the depiction of violence. 古書店アゼリアの死体 has something of all these characteristics.
AIZAWA Makoto (相澤真琴) comes to the seaside town of Hazaki after a run of bad luck, including losing her job. Her purpose there is to find a lonely beach where she can shout "Bakayarou" ("You bloody idiot") at the sea, to work off tension. The sea after all cannot answer back, she thinks. But then a drowned body washes up at her feet. The victim seems to be a young man of the influential MAEDA 前田 family, who disappeared twelve years ago as a teenager. The inspector who investigated that disappearance had been called off by his superiors, doubtless at the request of the boy's aunt Machiko (真知子) a wealthy and ruthless businesswoman. Now Machiko is pressing to have the drowned victim recognised as the missing boy; but is it really him?
Aizawa finds a new job in the shop of MAEDA Beniko (紅子), another influential figure in the town, though in her case that influence is mostly in the form of debts of gratitude owed for various kinds of help. Beniko's business skills had founded the family's fortunes; but now she busies herself with her hobby, a second hand bookshop specialising in romance books. When we meet her, she is driving an unwanted customer out of her shop, a detective story fan, who had found a romance written by a crime author he had been searching for. Beniko refuses to sell it to him. As someone who reads a fair few mysteries and more or less no romances, this might suggest that I am not the intended audience for the book either. There is some talk about romance literature (and an appendix, explaining the works that are casually referred to in the novel), and this might lead us to expect a romance plot. There is a romance plotline in the book, following the Pride and Prejudice model of initial hostility and mutual irritation; but this actually takes up very few pages.
Most of the time, the book is either mystery or social comedy. Often, the narrative style suggests that Wakatake got her ideas of storytelling from Japanese television dramas: in particular, she has a tendency to cut between scenes for drama or humorous effect. Apparently this is the second of several books set in the same town, like some British (e. g. Colin Watson's Flaxborough novels) and American series.
As to the mystery, the plot is complicated, unfolding as a series of smaller mysteries, and while some parts are fairly well clued, others are left as revelations that the reader could not be expected to guess.