Hi Eric,
Thanks for sharing your music. I'm glad you're having fun composing!
I recommend a book called "Composing Music" by William Russo.
http://www.amazon.ca/Composing-Music-A-New-Approach/dp/0226732169
In the book he goes through lots of different aspects of composition, from writing melodies, to form, to variation, to harmony and more.
It looks like you're getting a good grasp on harmony yourself! I have a challenge for you in the chamber piece. Give your melodies rhythms that stand out against the accompaniment. For example, the oboe melody in the last section. Melodies that begin or make changes on an off beat can be very effective.
All the best,
David