A dissertation is a document produced by a student in pursuit of an advanced degree. A thesis is similar but is usually shorter and done as part of a masters program. A dissertation is generally a comprehensive study of a narrow specialization. It should take into account all significant writings on the topic. Writers of dissertations are expected to contribute some original research and thinking, and they must not duplicate existing research – or research in progress. So in general, dissertations are not good for background material, but they can be very helpful once you know a lot about a topic, to extend your knowledge and fill gaps in it.
Dissertations can be found in several ways. The largest index to dissertations is now known as Proquest Dissertations & Theses, but was long known as Dissertation Abstracts. It is a very large, mutildisciplinary index to dissertations published in American universities since 1861. Proquest also owns most of the dissertations, and will sell them for a fee ranging from $37 to $75. The database includes over 2 million dissertations. It is searchable online through the Ritter Library database pages.
Dissertations in music since about 1967 can also be found in RILM Abstracts. Music Index appears to include dissertations, but on closer examination, they turn out to actually be citations to Dissertation Abstracts. So they can be helpful in locating dissertations, but you have to be careful not to take citations straight from Music Index.
Obtaining copies of dissertations is sometimes challegning, but it can be done. You can try interlibrary loan through Ritter Library or purchasing from Proquest. Please ask a librarian if you need any help with this process.