Here’s a great resource for those who don’t feel like spending your life’s fortune on the Kennan, White, or Forsyth texts. Best of all, this online database contains audio files that exemplify various orchestral scoring techniques from the repertoire. Very cool!
This site is part of a larger project called “The Sound Exchange,” that makes orchestral resources available online. The project is headed up by the Philharmonia Orchestra under Esa-Pekka Salonen (whose music has come up, variously, in studio class, and was featured prominently in a recent iPad advertisement)
Be sure to supplement your open-source learning diet with generous portions of IMSLP scores of Stravinsky, Rimsky, Mahler, Holst, and Vaughan-Williams, Bartók, and/or Lutosławski (just kidding, Lutosławski isn’t on IMSLP).