The Penguin Guide is pretty good for choosing specific recordings, but is not that great a guide for determining which specific piece is significant or a good introduction to a particular composer or type of music. It is unfortunate that the Penguin Guide has not been updated for more that a dozen years or so. But, some other people have used the Penguin ratings for identifying the better recordings that can be bought from their site. The very best recordings that are contained in the Penguin Guide earn a "Rosette." You can find lists of Rosette winners by googling Penguin rosette. This particular website has Rosette winners categorized by different musical eras which is quite helpful: