After booking an impressive return last year, activist investor William Ackman is upping his stake in a major bookseller.

Ackman’s hedge fund, Pershing Square Capital Management, returned 22% last year, Bloomberg News reports. The $4 billion fund hit it big on bond isurers MBIA Inc. and Ambac Financial Group, as well as on companies less stung by the subprime and credit crises like McDonalds Corp. and Ceridian Corp.

Not everything came up roses for Ackman in 2007: His investment in retail giant Target Corp. lost almost half its value last year.

Pershing Square has also boosted its stake in Borders Group over the past several weeks. The hedge fund, which has said it is betting on a turnaround at the second-largest bookseller in the U.S. engineered by incoming CEO George Jones, now owns 22% of the company. In November, its stake stood at 17%.