I am presently composing something for lever harp. I can recommend Anonymous’s Variations on a Theme of Mozart, edited by LeDentu or Megevand. Also, Salzedo’s pieces in the Art of Modulating are all diatonic and adaptable.
I believe most of Andres’ Espices can be played on lever harp and they contain some technical and rhythmic challenges. Alfredo Rolando Ortiz’s books are quite challenging and represent an important harp tradition. There does need to be more advanced art music written for lever harp. I’m planning to place a request in the American Composers’ Forum publication for challenging new art music written for lever harp, and plan to write a sonata for lever harp.
Barbara Brundage of Seaside Press has done some advanced level transcriptions of classical works for lever harp. Definitely worth checking out all of her stuff.
There is a series of Classical Pieces published by Billaudot the has some very advanced pieces (Scarlatti, JC Bach, Naderman, Godefroid, Chopin) edited by Odette Le Dentu and Dominig Bouchaud.
I have written a suite of three short pieces for lever harp (Spring in Vancouver, Fall in Vancouver, and The Downstairs Spider). They are at an intermediate to difficult level. If you like, I could scan a bit of each one and you could see whether you would be interested. Each time I have performed them, they have been very well received.