Can anyone suggest me good books for improving knowledge (and performance) of 19th and 20th century music?
The Memoirs of Hector Berlioz. Everyman's Library Edition is excellent.
This is an amazing book, not a history book, but a book that describes in vivid detail the emotions that inspired Berlioz's music, and gives a detailed account of what his life was like as a major composer in 19th century Europe. Most importantly, it is a very entertaining read.