No book or course on orchestration can actually replace the invaluable lesson you can learn by talking to and/or observing musicians who are playing your music. Whenever you have the chance to work with musicians, try to find time to have a little chat with them in private. Most musicians are very willing to share their concerns about certain difficulties, give you ideas on how to improve by keeping the musical intention intact etc.
Also, observing your musicians while they’re playing is very helpful as you can quite easily spot when they’re struggling to pull something off in the way you want it and which passages can be played without much trouble. I’ve learned a lot of things that have never been mentioned in any book on orchestration just by talking to musicians or simply by just observing what happens when you music actually gets played by real people.