This book is a sort of chronologically arranged encyclopedia of entries for what is colloquially termed 'classical' music. In the strict sense, not all of the music represented here is classical, but in the more common use of the term, these are the kinds of music that one finds on the 'classical' radio station.
Author David Dubal arranges things into five primary time periods. The first is Medieval/Renaissance/Elizabethan. This includes some of the earliest known named composers (often music was anonymously composed prior to this period). It is a lesser known period, and much of the composition of the time focused upon church and sacred composition. Most of the composers listed in this period (Tallis, Palestrina, Lassus, etc.) are notable church composers, but there are secular and folk-variations that also appear among their body of work.
The second time period is the Baroque, a time of increasing secular composition, although once again much of the music of the time was intended for church performance. The giants of the period include Handel and Bach, both noted for religious composition. Handel's 'Messiah' continues to be a crowd-pleaser, even though as an oratorio it wasn't original performed in churches. J.S. Bach's body of composition is so vast (and even what we have is incomplete) that rare is the Christian church in the world that does not have some of his work in their hymnals or service music.
The third time period is the most appropriately called Classical music. This includes the giants of Beethoven, Hadyn and Mozart. Dubal also includes Gluck in this major listing. Here composition includes pieces for the church, but the bulk of the material that is well known is not sacred; Beethoven's concertos and symphonies are most likely his best known (and possibly best) works, and Mozart's operas continue to captivate.
In the second and third sections, Dubal breaks his discussion neatly into larger essays about the major figures, and smaller biographical entries of the minor (or perhaps the term 'less major' is more appropriate, since 'minor' doesn't always apply) composers of the period. In the first section, there is no division between major and minor. In the fourth and fifth section, Romantic and Modern respectively, again the division line is more blurry. One can dispute the division lines, too - Vivaldi might be more aptly included among the 'majors' in the Baroque listings, whereas Gluck might not be one of the 'majors' in Classicism. With Romantic and Modern composers, the sheer numbers make divisions difficult to come by, and give such a wide range even within the periods. Romantics include Wagner, Tchaikovsky, Paganini, Brahms, Elgar, Puccini, Mahler, Debussy - and this isn't even half the 'majors' listed. Moderns include Britten, Berg, Gershwin, R.V. Williams, Ravel, Stravinsky, and many, many more.
This book includes biographical details, composition details, and suggestions for particular recordings (which includes performers, recording labels and production numbers for ease of location). There is a name index at the end for those searching in non-chronological fashion.
This is a guide written with the non-specialist in mind, so there is not a lot of technhical information included, but it does also serve as a good catalog for those who have had musical training (not everyone needs the 20-volume set of Groves).