Langen took over Gretor's grandiose apartment on the Boulevard Malesherbes, along with expensive furniture and an extensive collection of images (including some, it was said, of dubious authenticity).
In addition to Scandinavian authors such as Bjørnstjerne Bjørnson, Georg Brandes, and Sven Lange, Langen also expanded into contemporary French and German literature.
He was especially successful with paperback editions with signature bindings of, at first primarily French, artists such as Jules Chéret, Théophile Alexandre Steinlen, and especially Thomas Theodor Heine.
In the years that followed, he was able to publish works by Heinrich Mann, Henrik Ibsen, Marcel Prévost and Verner von Heidenstam.
He remained in exile until 1903, when George, King of Saxony, pardoned him on the condition of paying a "reprimand sum" in the amount of 20,000 Marks.
Two years later, as a result of his employees demanding a share of the profits, Simplicissimus became a "Gesellschaft mit beschränkter Haftung" (GmbH), the German equivalent of a limited liability company (LLC).
In April, 1909, he died from the effects of a severe middle ear infection; apparently contracted when he drove in an open car to the landing site of the zeppelin LZ1, near Lake Constance, on an unusually windy day.
Heine and Bruno Paul were his most important book artists (designing the covers, dust jackets, vignettes, and illustrations), but both worked quite differently.
Other illustrators included Ferdinand von Reznicek, Eduard Thöny and the Norwegian Olaf Gulbransson who joined the firm in 1902 and whose minimalist drawing style eventually became as indispensable for Simplicissimus as Heine's.
Several writers (including Henrik Ibsen, Jakob Wasserman, and Ludwig Thoma) briefly published with Langen but then returned to S. Fischer, whose earlier entry into the market (in 1886) proved insurmountable.