One of the things that has helped me compose my own material over the years, has been to listen to commercial recordings of other people's material and then try to come up with the melody, the 'voice leadings and harmonies' and the chord progressions for the song. Then I give it to a professional jazz musician and have them critique the leadings and the chords (normally we agree the melody is OK). I have done this many times and have learnt much about what composers before me have written and how today's musicians interpret and play these 'classics'/'standards'. I can't say how much doing this has helped me with my writing, arranging, and orchestrations. While studying scores is also important, in my opinion, the more active 'doing' of the transcription process is an eye opening and wonderful way to learn from others. And in fact, for some, doing transcription arrangements of other's material can be a profession in and of itself.
So here is one I decided to work on myself today, which you may want to try for yourself. I pick those which as far as I know are not in any of the three REAL BOOKS - since most professional musicians have access to these already. So try this one WITHOUT looking for lead sheets, chord sheets, or any other 'temptations' online. Just listen to the attached youtube film clip.
"It Only Happens When I Dance With You" written by Irving Berlin appears in Easter Parade (1948) and sung in the clip by Judy Garland. And we do this song because today is Judy's birthday (born in 1922). See if doing this 'sounds' like it does in the film and then think of what doing the transcription has taught you.