Look up history, theory, composers and compositions in the New Grove Dictionary online. It's a good idea to get familiar with the print version first.
Oxford History of Western Music
Read Taruskin's insights into history and the great composers - the same text as the book, but a searchable e-book format!
The Nelson Library has a wide range of books on Music History.
Here are some sample subject searches, you can also search for a composer's name as a keyword in the Discovery catalogue.
If you wish to browse the library shelves you may find these numbers helpful:
780-789 Music
780.285 Computer applications
780.9 History of music
781 Theory of music
781.3 Composition
781.34 Computer composition
781.37-781.38 Arranging
781.542 Film music
782.14 Musicals
783 Music for voice
784 Popular music
785 Instrumental music
785.42 Jazz
786 Keyboard, mechanical, electronic, & percussion instruments
787 Stringed instruments
787.87 Guitar
788 Wind instruments
Or search the Catalogue!
Holding the note : writing on music by 
Loading the Silence: Australian Sound Art in the Post-Digital Age by 
Sound bites : following the bendy path of classical music from Ancient Greece to today by 
Music : the definitive visual history by 
The Cambridge Companion to Rhythm by 
The other classical musics : fifteen great traditions by 
