This brief novel from 1892 is written in the form of diary entries by a clerk in some sort of financial firm who believes his diary will be studied some day in the manner of Samuel Pepys. The joke is that nothing of any particular interest every happens to the character. To give you an idea of the general level of humor at the expense of the narrator, our hero's name is Mr. Pooter.
Grossmith's novel is an interesting contrast with the writings of his more or less contemporary P. G. Wodehouse. Wodehouse made fun of the silliness of the English upper class while Grossmith mocks the pretentious of the English middle class. While Wodehouse is sending up his subjects he clearly considers them charming and funny and therefore his writing is charming and funny, but Grossmith looks down on his subjects and considers them fatuous and boring and as a result his writing is kind of fatuous and boring.