Best single volume guide to classical music

I would start with something simple, then go for ’best’.

https://www.google.com/search?q=guide+to+classical+music&rlz=1C1JSBI_enUS1081US1081&oq=guide+to+clas&gs_lcrp=EgZjaHJvbWUqBwgAEAAYgAQyBwgAEAAYgAQyBggBEEUYOTIHCAIQABiABDIHCAMQABiABDIHCAQQABiABDIGCAUQRRg8MgYIBhBFGDwyBggHEEUYPKgCALACAA&sourceid=chrome&ie=UTF-8

Maybe not exactly what you’re looking for but “Year of Wonder, Classical Music For Every Day” by Clemency Burton-Hill presents a classical work each day with a back story. There is a companion playlist (unfortunately on Spotify). 

Funny that you pose this question as I am looking at what I have been using as I read your post. In any event you may want to check out Penguin Guide to the 1000 Best Classical recordings.  What I have is a 2011 version so perhaps there is a newer version.  The penguin guides used to be very well regarded but now the older versions are dated unless you collect records in which case they still would be useful.

Funny that you pose this question as I am looking at what I have been using as I read your post. In any event you may want to check out Penguin Guide to the 1000 Best Classical recordings.  What I have is a 2011 version so perhaps there is a newer version.  The penguin guides used to be very well regarded but now the older versions are dated unless you collect records in which case they still would be useful.

Greene's Biographical Encyclopedia of Composers by David Mason Greene. Best classical music bathroom reading of all time. Not in print apparently but not at all expensive used.