This page offers advice about choosing a research topic using reference books and collections of primary resources.
Choose your research topic wisely. My advice: select a broad area that interests you but do not commit to a specific project until you (1) read some overviews--maybe in a specialized encyclopedia, maybe in a monograph--about the subject,(2) determine that there are sufficient secondary materials, and, probably most important,(3) that there are primary documents available in a language you read.
It is often a useful approach to begin with available primary documents and frame your questions on the available resources!
Consider basing your project on one or more primary documents rather than looking for documents to support a project based on readings in secondary materials.