Monthly record guide

POP ROCK Billy Joel: ''An Innocent Man.'' (Columbia QC 38837) - Here's classic '50s, early '60s rock a la Billy Joel. Not a smothering imitation but an interpretation - and an excellent one. And that's good news. Last year's ''The Nylon Curtain'' was something of a modest effort, although one tune, ''Allentown ,'' has achieved near-greatness. (Joel's telling blend of music and lyrics was transfixing in its description of a Pennsylvania factory town.) Though ''Innocent Man'' songs all have an easily labeled sound to them, Joel doesn't hogtie his music. There is some musical quotation - ''Uptown Girl,'' for instance, reminds one instantly of the Four Seasons. But the early rock styles are more a departure point from which he creates his music. And the lyrics, though at first appearing to mimic the naivete of some early rock, sometimes do much more. Like ''An Innocent Man,'' the quiet title song, with its message of buoyancy. And I can't remember Billy Joel's voice being so flexible. By now, with its airplay on the radio, ''Tell Her About It,'' and its friendly advice to a young man how he should treat his lady friend, sounds like a pop standard. This is a credit not just to Joel's songwriting ability but his skill here at capturing the feel of this classic rock. I wouldn't be surprised, though, if ''This Night,'' a charming rock ballad with a soaring chorus borrowed from Beethoven, will be next to gain favor. Then there's ''Easy Money,'' where the music so splendidly illustrates the seedy lyrics.$90David Hugh Smith