Introduction to Modernism (1860-1970) 

                           (Held between World war 1 and early 1970's)
What is Modernism?
 Definition: Modern character of quality,thought,expression technique.
                             A style or movement in the arts that aims to break with classical and traditional form.
I found out another definition from a website: A self conscious break with the past and a search for new forms of expression.

 So modernism is basically a difficult topic to discuss for everyone because everyone has their own point of view about art and its fundamentals (music,sculpture,dance,technological innovations and much more). But here I am talking about what it is and how it started.  

the word ‘modern’ was used to refer generically to the contemporaneous; all art is modern at the time it is made. In his Il Libro dell'Arte (translated as ‘The Craftsman’s Handbook’) written in the early 15th century, the Italian writer and painter Cennino Cennini explains that Giotto made painting ‘modern’ [see BIBLIOGRAPHY]. Giorgio Vasari writing in the 16th century, refers to the art of his own period as ‘modern.’

                         http://arthistoryresources.net/modernism/roots.html

So there is not much space to disagree with the poitn that all art is modern at the time it is made. Its quite interesting thats why we they termed it as modernism.
 I found these info's ineteresting:
One of the earliest Modernist painters was Edouard Manet. Arriving on the art scene in between Realism and Impressionism, Manet painted what was before him; a perfect example is his "Olympia" (1863) in which he painted a French courtesan. This inflamed audiences, not only because of the subject matter but also because Manet painted her as she was -- bold, brash and unashamed. Predecessors would never have chosen this subject, but if they had, it would have been a lovely, classical nude. This is the first of many examples where artists broke from traditionally accepted standards.
Impressionism came next, where artists were criticized for not painting actual subjects, but just impressions -- in reality, they were distinguishing between what we see visually versus how our brains process that stimuli, showing the impact scientific knowledge had on the arts [source: The Art Story]. With this, the floodgate of avant-garde movements broke loose and the "isms" poured forth. Post-Impressionism flattened space; Fauvism played fast and loose with color; and Cubism shattered traditional ideas of perspective. Artists took every opportunity to break free from accepted teachings and techniques. Dada is a great example of modernism. Affected by almost every avant-garde movement to come before it, Dada was so multifaceted that even Dadaists couldn't agree with themselves. According to artist and author Steven Farthing, Dada was about freedom of expression and anti-leadership so it was an almost anything-goes philosophy on art.   http://curiosity.discovery.com/question/what-is-modernism

Books I am interested in for finding more clear about modernism.
http://www.publishersweekly.com/pw/by-topic/industry-news/tip-sheet/article/58283-the-10-best-modernist-books.html