Over the past 12 years, longtime banking analyst Mike Mayo has issued numerous calls to sell bank stocks, a rarity in a system where nearly all stocks are rated buy or hold. His negative ratings have frequently gotten him in trouble with banks, clients and his own bosses, who didn't want to alienate those companies. In this excerpt from his new book, "Exile on Wall Street," Mr. Mayo gives an inside view of the fights, the scolding and the threatening phone calls he received as a result of yelling "sell"—and offers a proposal to fix the banking sector.

Taking a negative position doesn't win you many friends in the banking sector. I've worked as a bank analyst for the past 20 years, where my job is to study publicly traded financial firms and decide which ones would make the best investments. This research goes out to institutional investors: mutual fund companies, university endowments, public-employee retirement funds, hedge funds, and other organizations with large amounts of money. But for about the past decade, especially the past five years or so, most big banks haven't been good investments. In fact, they've been terrible investments, down 50%, 60%, 70% or more.

[Be sure to read all the way to:] What we need is a better version of capitalism. ...