Dasvon

Many of my faves are already mentioned. I'd suggest an area you could research of writers who really applied music theory into creating that "new sound" yet still had a bead on what would be popular were those that resided in the "Brill Building" Search "Brill Building" for information that would take you from the 40's up through the 70's for writers like Neil Sadaka, Laura Nyro, Carol King, Neil Diamond, Jimmy Webb, Burt Bacharach and others. There is where you got those jazzy chord harmonies of the 5th Dimensions and other groups. Speaking of musicals as referenced in a post above, HAIR is an excellent choice to review for harmonies and music theory application.