Know any good books or internet guides on composing for big band?

While books are great (and the one linked above looks like a good one to use) my suggestion would be to get a hold of some of Ellington's and Miller's scores and study them. You can usually get individual ("extra") scores for about $10 each, and libraries may have some scores on hand as well. You could definitely use a book to supplement this study, but you might as well learn straight from the masters. Take a score and just pick it apart...analyze the melodies, harmonies, rhythms, form, everything you can possibly think of. Then use what you've learned in your own music!