Fiction
La damisel·la santa - 1894
Els sots feréstecs - 1901
Les multituds - 1906
Llibre d'històries - 1909

Painting history
La pintura gótica catalana
 del siglo XV - 1892
Història documental de la
 pintura catalana - 1905
Estudis diversos sobre
 pintors del segle XVIII:
 Viladomat
 Els Tremulles
 El dibuixant païsista Lluís
  Rigalt - 1900
 Flaugier

Art critique
Etapes estètiques - 1910

Life:
Born in Barcelona in the year 1855.
He studied in the Barcelona Catholic Seminar, but due to the dead of his parents, from 1872 he take in charge the familial dry cleaner's business. From 1881, he is redactor in the magazine "L'Avenç". From 1882 as art redactor of the journal "La Vanguardia" he start his activity as theoretic of Modernisme (Art Nouveau) together with other artists like Rusiñol and Casas. In this way, he took part in the Festes Modernistes (Art Nouveau fests) in Sitges. A result of this first part of his work is the novel "La damisel.la santa" in 1894. From 1899 he is the chief redactor of the journal "La Veu de Catalunya".
In 1901he wrote his most known novel "Els sots feréstecs" who made a strong effect on literary modernist circles and is considered one of the masterpieces of Catalan literary Art Nouveau, and at the same time, one of the first examples of rural novel developed during the Art Nouveau period. 
He was also collaborator of other cultural magazines as "Hispania", Catalònia", "Empori", "L'Esquella de la Torratxa" and others.
Casellas also wrote two collection of tales, "Les multituds" on collective psychology in 1906 and "Llibre d'histories" with an ironic view on social customs and manners in 1909.
Finally, Casellas wrote also some non published books on Catalan painting history.
From 1909, after the "Setmana tràgica"  - a short revolutionary period held in Barcelona during the levies for the war in Morocco - he start a depressing period who was ended with the suicide in 1910 in Sant Joan de les Abadesses - Ripollès -.