Harper's Dictionary of Music

As a music grad student, reference is everything. Everything from theory terms to composer bios, from instrument descriptions to dynamic markings, HarperCollins does an excellent job at detail -- at the same time making it concise enough to fit into your average knapsack. I've used the 3rd edition all through my college career and it's safe to say the approach is sophisticated enough to be used by those well-versed in the field, while maintaining word-usage that is basic enough for your average 'joe' to understand. For this price range, I strongly recommend this to be your every companion should you delve into music in any way.