The biggest challenge I have come across as a composer has been the technical limits of instruments I don’t play, and that list is fairly lengthy. While orchestration books help tremendously, sometimes you need to listen to musical examples, look at seating charts or fingering charts, or look up the harmonic series. These are all online at a wonderful page called The Orchestra, A User’s Manual.
http://www.mti.dmu.ac.uk/~ahugill/manual/