Harmony

There are quite a few good textbooks out there:

I have found the following useful (all currently available):

Edward Aldwell and Carl Schachter: Harmony and voice leading (dense)
Bruce Benward's, Music in theory and practice (a great start)
Alex Burnard: Harmony and Composition (old but still fresh)
J. Chailley and H. Chaillan: Théorie Complète (Comprehensive French text)
Diether de la Motte: The Study of Harmony: An Historical Perspective 
Théodore Dubois: Traité d'Harmonie (The grandfather of French harmony texts)
Paul Hindemith: Traditional Harmony (Conservative and useful)
Vincent Persichetti: Twentieth-Century Harmony (Craft techniques for serious composition)
Walter Piston and Mark Devoto: Harmony (Tonicization started here!)

For general information see the New Grove Online (when you're in the library or on your student access account.
Dolmetsch Online Dictionary  or Music Theory & History are relatively accessible and generally accurate places to look for things quickly if you don't have Grove online access.

Part-Writing - a summary from The Harmonic Dimension - Gary White, Wm. C. Brown Publishers

(Four part style is a highly refined procedure which doesn't automatically reflect the real world. Nevertheless, knowing the conventions does reflect an understanding of voicing and leading)