Paul Simon has always been a restless artist, going all the way back to “Cecilia,” the percussion-driven world music sing-along he slid onto Simon & Garfunkel’s Bridge Over Troubled Water. But his excursions over the past 15 years — including a Broadway musical and a collaboration with Brian Eno — were misguided and not very good. So Beautiful or So What, his first album in five years, skips any sort of big concept, keeps the focus on songs, and turns out to be his best work in 20 years. And it all indeed comes down to the songs. Simon hasn’t made this straightforward of an album since 1983’s Hearts and Bones. “Getting Ready For Christmas Day,” “The Afterlife,” and “Love Is Eternal Sacred Light” are based on relatively unfussy songwriting models. Of course Simon can’t help but sprinkle some cuts with subtle samples or global ornaments. But they’re mere decoration on songs that don’t really need them.