The Oxford Companion to Music

I've had a copy of the tenth edition of The Oxford Companion to Music close at hand for thirty some years. It's the best single resource for information and insight into "classical" music I've ever come across, and is delightfully entertaining as well, due to the wonderful writing of Percy A. Scholes.

This opening paragraph of the entry on tempo is a great example of what is, to me, some of the best writing on music there is.

Tempo usually means 'speed'. Upon the choice of the best speed the effect of music greatly depends. Every composition may be said to have its correct tempo, but this is not capable of being minutely fixed without scope or variation, as to some extent circumstantial factors enter, such as the character of the instrument used (e.g. organs may greatly differ in their effect), and the size and reverberation of the room (a very reverberant room requiring a slower tempo if the music is to 'tell'). Moreover, the general character of the interpretation decided upon may affect the tempo: one performer may consider that a particular piece will be most effective if every detail be made clear (calling for a slower tempo) and another that it will be most effective if treated in a 'broad' style calling for a quicker tempo; and both these interpretations may be good ones. Further, a highly rhythmic performance at a slower tempo may give the impression of being quicker than a really quicker one with less rhythmic life. In fact, what matters is not the tempo the performer actually adopts but the tempo that the listener is led to imagine he is hearing, for whilst in science things are what they are, in art things are what they seem.