English instructions are in the front of each volume. Spectra are represented graphically. Indexed by name, molecular weight, CAS Registry Number, and peak intensities. This is a three volume set. The Contents pages give a more detailed breakdown of the contents of all the volumes.
Importance is a combination of uniqueness and abundance.
Pages vi-vii in volume 1 explain of how this is calculated. Spectra RS189.P425 1992. Descriptions of these are located in the table of contents. There is a chemical name index in each volume. A rich compendium of information, including mass spectral data for several thousand substances.