Peter Child's 2000 book Modernism, in The New Critical Idiom series, is exactly what it sets out to be: a brief but diligent overview of modernism, the literary/cultural movement that arose in response to changing times at the turn of the century, signaled by increasing industrialism, urban life, technology, and eventually by the first World War.
As the world grew ever more complex, the capacity for individuals to understand the world diminished. It can be seen then why a prevailing philosophy of realism, in which it's assumed that all members of a rural society possess a basic, common-sense grasp of the world, should have been compelled to give way to a more internalized and subjective response, inherent to each individual, to the complicated urban world. Childs' vision of modernism is less as an artistic movement than an uneasily reacting society in crisis. Not that the new, complicated world was necessarily seen as dystopian, but that it was perplexing, demanding an unprecedented degree of sophistication.
The literary technique known as stream of consciousness is probably the most familiar characteristic trait of modernism. Stream of consciousness writing perhaps became inevitable as people moved out of the wide open country and retreated inside their own heads. Of no less importance was the need many modernist writers found to try to impose order on the new chaos through the imposition of systems or architectures of symbols to provide structure for their works: structure which now seemed lost in the world around them. The reassuring internal logic of these symbol-systems served in a role analogous to that formerly paid by stock-characters plowing through plot-heavy dramas in the preceding era of Victorian realism.