Schoenberg Op.19 (Very short 6 pieces that are pretty easy, but it's hard to make it sound coherent and actually meaningful).
Webern Op.27 Variations
Scriabin 5 Preludes Op.74
Prokofiev 4 Pieces Op.4
Berg's Sonata if you are into a longer piece. (Not atonal but highly dissonant)
Ravel's Oiseau Tristes (Not atonal)
Bernstein Touches
Copland Passacaglia (Not atonal pretty sure)
Taneiff Prelude and Fugue (Not atonal, but decently dissonant)
Szymanowski Mazurkas Op.50
Carter Catenaires
These are all "modern" pieces on the easier side, majority of them are early 20th century, since the later half of that century consisted of a lot of difficult works (and also because of my limited knowledge on it)
Anyways, here's a list of super difficult modern pieces you may want to listen and maybe try to learn in the future:
Szymanowski - Sonata Op.36
Barber - Sonata Op.26
Prokofiev - Sonata Op.83
Feinberg - Sonata Op.13
Roslavets - Sonata No.1
Bartok - Etudes Op.18
Scriabin - Sonata Op.64
Ginastera - Sonata Op.22
Kapustin - Sonata Op.62 (Not atonal but modern and heavily jazz)