Jana Partners, described in various media reports as a hedge fund known for taking an activist role in companies in which it invests, acquired 6.59 million shares of Barnes & Noble, giving the firm an 11.6% stake in the company. News of the investment, made in a Securities & Exchange Commission filing Monday, resulted in an 18% boost in B&N’s stock price yesterday. Jana has an option to acquire another 250,000 shares.

The investment makes Jana the fourth largest stakeholder in B&N, following chairman Len Riggio, Liberty Media, which made roughly a $200 million investment after talks about possibly buying B&N, and Ron Burkle. Among the companies where Jana has agitated for change is the McGraw-Hill Cos. which is in the process of splitting the company in two. B&N has been exploring the possibility of spinning off its Nook group into some time of separate company.A decision is expected this spring.