A comic sociological fiction story by Connie Willis.
Sandra Foster is a sociologist trying to discover the origins of fads (using hair bobbing in the 1920s as a case study). This is no easy task, but it is made harder by the fact that she is employed to do this by HiTek, a large and clueless company which would be perfectly at home in a Dilbert strip. She also has to work with Flip, the highly incompetent office assistant, who throws all she touches into chaos, and any number of coworkers who are happily following the latest fads -- something Sandra views with some distress.
Most of Connie Willis' books are Science Fiction, and Bellwether itself is usually classified as SF. But it is not set in the future, doesn't have any fancy gadgets, no time traveling, and no fantastic happenings. It does come up with theories of what causes fads and what causes scientific discoveries that are somewhat unlikely (the fad explanation more so than the scientific discovery one), but the main hook of this book is the general perversity of human behaviour... particularly the behaviour of Flip.
Aside from the humour and the fast-moving story of the chaos that is Sandra's life, the third notable aspect of this book is a review of any number of historical fads and scientific discoveries. Connie Willis always researches her subject matter (she is one of the better writers of historical science fiction/fantasy); aside from Sandra's observations on any number of historical anecdotes and how they relate to her current problems, each chapter starts with a capsule review of a fad, from the 12th century to the 1990s. There is also some chaos theory involved; while it is important to the plot, there isn't any real detail given on the subject.
It's a very good book; it moves along quickly, it's funny, and it's interesting. On the other hand, the ending is fantastic out of proportion to the rest of the story, and the plot is not in strong evidence (although, if you can ignore that there isn't any strong direction to the story, it's very engrossing). The book is a great example of Connie Willis' writing style; if you haven't read any Connie Willis, you might do better to start on another book (I suggest Doomsday Book). If you are already a fan, you do need to read this book. It's a prime example of her humorous fiction.
Bellwether has been published on its own (originally in 1996, Bantam Spectra Books), but these days you might be more likely to find it in the collection Futures Imperfect: Three Short Novels.