“Like a comet that returns every 15 years or so, Daniel Barenboim is playing all 32 Beethoven piano sonatas again.” These days, the omnipresent Barenboim is better known as conductor than pianist, and most of his recent successes have been in the realm of Wagner opera, a far cry from the delicate complexity of Beethoven. David Patrick Stearns is intrigued by Barenboim’s continuing obsession with the sonatas, and also by the performer’s seemingly endless ability to rethink works he has played hundreds of times before.