Northern Sounds has the entire text of "Principles of Orchestration" by Rimsky-Korsakov online for free. Even better, the musical examples are presented as scores with audio!
http://www.northernsounds.com/forum/forumdisplay.php/77-Principles-of-Orchestration-Online
 
This was uploaded quite a few years ago but I thought I'd share it for anyone that might not be familiar with it. 
 

Terrific resource I was unaware of. ¡Muchas gracias!

Great recource, thanks Dave!