First you need to get a better ear: Copeland gently gives you the building blocks to comprehending the structure of music. He reveals both the form and the function of music.

You need to be able to recognise, hear and sing what you see on a score easily: this book offers an approach to sight singing.

A great introduction - and guide to using instruments in your compositions. Continiously interesting - everything but a boring lecture. A good extra companion for the Study of Orchestration.

Alternative...(just one of the more popular ones)

After basic harmony, get even more original - don't think this goes solely about serial music - this book will expand your style incredibely.

Texture and voice leading: the classic book on counterpoint is Fux's - Bach knew of it and approved it, and Haydn, Mozart and Beethoven (among others) studied from it.

An important step in creating your own style is understanding those of others, foremost by analysis. For those who are taking composing really serious.

If you also want to compose for movies: the basic guide (for further info, consult other listmania's: http://www.amazon.com/exec/obidos/tg/guides/guide-display/-/242URG66XLHBO/ref=cm_bg_lm/102-56130).

Next comes further reading: the Forsyth orchestration book gives info from a musicians point of view (N.B.: not completely up to date! - but still usefull).

The most complete next to Adler (which is twice as large) - though I'd advice to get the two beforementioned books first, because the Adler book recommends them (which is of course, the bible :-p).

Of historical as well as practical interest: a book of two of the greatest orchestrators.

Another major harmony book, but not the most detailed one.