Among his musical contributions are records of eighteen folk songs which formed the basis for Grieg 's opus 66 .
A large portion of the letter correspondence between Grieg and Beyer has been preserved and later published .
Among Beyer 's musical contributions are records of eighteen folk songs which formed the basis Grieg Grieg 's opus 66 .
A large portion of the letter correspondence between Grieg and Beyer has been preserved and later published .