Henderson, What Is Good Music? (chapters XIII and XVII).
Krehbiel, How to Listen to Music (chapter VIII).
II. Interpretation:
Coward, Choral Technique and Interpretation. One of the few really significant books on conducting. The author gives in a clear and practical way the principles on which his own successful work as a choral conductor was based.
Matthay, Musical Interpretation. A book for the musician in general, rather than for the conductor specifically; an excellent treatise and one that all musicians should read.
III. The Orchestra:
Lavignac, Music and Musicians (chapter II).
Mason, The Orchestral Instruments and What They Do.
Corder, The Orchestra and How to Write for It.
Prout, The Orchestra (two volumes).