#1
I'm looking for a book on arranging and harmonization, because I would like to start adding this kinda stuff to my own writing. I know a pretty good amount of theory, the constructions of cords, scales and modes, but I would like learn more about voice leading and other things symphonic composers use.
#2
You'd need to learn counterpoint, most arranging books require that you know counterpoint.

To learn counterpoint, most books/courses require you to know the classical way of writing melodies.

To write melodies, you need to know your theory. I'd recommend you know your theory inside out.

With that in mind, anything by Walter Piston, Kent Kennan or William Russo is brilliant.
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#3
I know counterpoint pretty well, and i am some what familiar with classical writing, but if there are any books that have it all i would love to find them.
#4
William Russo - Composing Music A New Approach (damn good)
William Russo - Composing for the Jazz Orchestra (it's actually about arranging, not composing)
Mark Levine - Jazz Theory Book (sort of helpfull)
Walter Piston - Harmony
Walter Piston - Orchestration
Kent Kennan - Counterpoint (has a section on arranging iirc)
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#5
So if you were to recommend only one, would it be the first? (I wish I could pick a couple up, but my budget isn't great)