Ok composing rank 5, the composing book, Instrumental Timbre, can be obtain through the reward of Exploration Handbook 12. I have no clue what that is.
For Musical Knowledge rank 6, the book is call The Soundbox: Heart of the Instrument, can be obtain by running Barri Adv 3
For Musical Knowledge rank 5, the book is call String Instruments: Hand-made Melody, it can be obtain by complete Exploration Handbook 14. I have no clue what this is either. <3 Good luck peeps of the musical world in Erinn <3