Méthode de harpe
This book does not have many interesting things for improvisation, just a few tidbits. At the end there are 15 prelude examples, …
L’Art de préluder sur la harpe
This was written to follow his harp method. More information about this method can be found here (harpmethods). In terms of improvisation …
L’art de préluder au piano op.94
An example-based treatise.