To understand modern art you have to understand Modernism, which is as much a writing movement which started with Zola, Balzac and the other realists, before morphing into --among other things Salman Rushdie.

Start with the Autobiography of Alice B. Toklas by Gertrude Stein, a memoir of the Cubists (Picasso and Braque) by a collector of their work told from the point of view of her life partner. Look at the painters she discusses, including Matisse and Cezanne, which mean you should probably get books out of the library about the Impressionists and Post-Impressionists as well. Then get books about the Bauhaus in Germany, and try to pick up a copy of Vassilly Kandinsky's Concerning the Spiritual in Art. You will see a whole different, but related approach to abstractionism and modern art happening at the same time as what Stein is describing in France. And I guess, since it's such a big subject, you will have to take it from there.

Two anecdotes from art history will help.. In the autobiography a young man told Gertrude, Gertrude, I used to love the old masters and hate modern art, but now I have come here and listened to the modern artists talking about their work I can't stand to look at the old masters either. In Ad Reinhart's Art Comics he has a cartoon of someone laughing at a canvas and asking, "What does it represent?" Suddenly the painting comes to life with a face and sketchy arms, one of which points at the man accusingly and asks, "What do YOU represent?"