Have you looked online for commentary in JSTOR?
For example, you can read Julian Rushton's review of the String Quartet by Jonathan Harvey. A great way to find your way around the piece! Click HERE. (Direct access from within the RCS for our staff and students - please use your institutional login if you're off-campus.)
Use the Advanced Search for best results, and make sure you choose the option for "only material I can access". (That avoids the frustration of getting links to material that the RCS doesn't subscribe to!)We subscribe to lots of other premium online resources, too: Oxford Music Online (Grove Dictionary of Music), RILM (abstracts of music articles etc), lots of electronic journals, and streamed music and videos.
The RCS Library and IT website has pages for electronic databases and electronic journals. Take a look today! It might just save you a chilly trip to the library while you're writing your essay.