Harmony
Schoenberg - Harmonielehre (Theory of Harmony) (must have)
Schoenberg - Structural Functions of Harmony (must have)
Gauldin - Harmonic Pratice in Tonal Music
Kostka & Payne - Tonal Harmony
Rameau - Treaitse on Harmony
Persichetti - 20th Century Harmony (must have)
Straus - Post-tonal Theory (Not quite harmony

) (must have)
Counterpoint
Kent Kennan - Counterpoint (must have)
Fux - Gradus ad Parnassum
Mann - Study of Fugue
Schoenerg - Preliminary Exercises in Counterpoint (must have - rare to find, out of print)
Jeppersen - Counterpoint in the Polyphonic Stlyle of 16th Century
Form
Schoenberg - Fundamentals of Musical Composition (must have)
Orchestration
Adler - Study of Orchestration (must have)
Korsakov - Principles of Orchestration
Berlioz - Treatise on Instrumentation
That's all I can think of now.