Barbara Ehrenreich wants to make clear that she is not a spoilsport.

“No one can call me a sourpuss,” she declared. “I have a big foot in the joy camp.”

She is the author of “Dancing in the Streets,” a history of “collective joy,” she notes, and a lot of fun at parties. So her new book, “Bright-sided: How the Relentless Promotion of Positive Thinking Has Undermined America,” should not be mistaken for a curmudgeonly rant. It is serious social history.

Many of the 17 books that Ms. Ehrenreich has written during the past three and half decades have taken her into alien worlds. In her fantastically successful 2001 book, “Nickel and Dimed,” for example, she details her experience of trying to get by on the salary of an unskilled, minimum-wage worker. By contrast, this newest volume is based on her stay in a world that she became intimately familiar with: the smiley-faced, pink-ribboned, positive-thinking culture that surrounds breast cancer patients.

Ms. Ehrenreich found out she had the disease in 2000, and the news left her dazed, fearful and, most of all, angry. What she found when she sought information and support, however, was cheerfulness, and that shocked her.