The most useful composer resource I've ever found

[Dave Dexter]

https://isfee.music.indiana.edu/

Barring experienced humans willing to put up with my stupid questions, this has helped me more than anything else. Almost all major instruments a composer is likely to be writing for, with video and audio detailing their ranges, idiosyncrasies, notations, differing timbres and notable passages from the repertoire. All for free. It was invaluable to me as I started and I still use it regularly for the confidence it gives my work. It went offline a while ago when Flash stopped being supported and then came back. 

It doesn't tackle orchestration or composition at all, but it's about the most objective treatment of this messy field that I think exists. It's one thing to read the range of a trombone, it's another to see a player struggle with high notes and realise you shouldn't risk it. 

[gregrorio j]

Hey Dave, your example of the trombone reminds me (iirc) of when Stravinsky was responding to the difficulty of the solo bassoon part in 'The Rite of Spring', and how hard it would be to play in that high register. He said something like, "That's what I want to hear - the player struggling to play the notes! :)))

[John Driscoll]

Great! I also find the VSL instruments pages super helpful and surprisingly detailed: https://www.vsl.info/en/academy/index