Complex sentence: This was a polished and confessional album that was eventually viewed as one of his best works, but that marked a lull in his commercial popularity; both the album and the lead single, "Allergies," missed the American Top 40.
Split into simpler sentences: This was a polished and confessional album that was eventually viewed as one of his best works, but the album didn't sell well when it was released. This marked a low point in Simon's commercial popularity; both the album and the lead single, "Allergies," missed the American Top 40.