I don’t know much about Polish art, but have at least heard that Polish poster art is stunning. It has been used for political purposes (those of resistance movements, for instance) and entertainment (notably, movies).
To take my first steps into Polish poster art I haphazardly searched and found The Legacy Of Polish Poster Design. Andrea Austoni gives a very good history of the various periods of Polish poster art with good illustrations. Let’s quote him to give some background for Polish poster art.
The Polish film poster is artist-driven, not studio-driven. It is more akin to fine art than commercial art. It is painterly rather than graphic. What sets the Polish poster apart from what we’re used to see in the West is a general disregard for the demands of the big studios. The artists requested and received complete artistic freedom and created powerful imagery inspired by the movies without actually showing them: no star headshots, no movie stills, no necessary direct connection to the title.
The poster had just been born in France at the hand of Jules Cheret following the invention of color lithography. Influenced by the achievements of the French masters of this new art form, Henri de Toulouse-Lautrec above all, these Polish artists chose the poster as the new medium of expression. They were well respected, connected with the Academy Of Fine Arts and members of the Society of Polish Artists “Sztuka” (Art). The poster thus became acceptable as a form of art.

Jan Bulas – Symphonic Concert (1910). A poster inspired by Expressionism.

Wojciech Jastrzebowski – Swoszowice Health resort near Krakow (1907). A fine example of Japanism.
Secession, Japonisme and modernist styles like Expressionism were mixed with traditional elements of symbolism and national folklore.
You’ll find more examples of earlier poster art in Poland at this site: The Legacy of Polish Posters.
Besides the Austoni site, here’s a fun one for Polish posters of movies: 50 Incredible Film Posters From Poland.Polish poster art is not just about movies. These posters could be about musical events, political propaganda, auctions, or about health spas.
Here are a couple more posters from 10 Posters from Poland’s Golden Age of Graphic Design

Plyty Gramofonowe Tonpress, original Polish poster by Lech Majewski c. 1978

(Death in Venice), original Polish film poster by Maria Ihnatowicz c. 1971
The high quality and extent of Polish poster art is generally unknown, and is thus also a promising opportunity for investment and art speculation.