 Some of the most common resources that we see used or cited in our nominations bibliographies include standard architectural reference guides, not to make any recommendations, but one of the most common is Virginia and Lee McAllister's Field Guide to American Houses. Another would be long-strets, the buildings of Main Street. These are often used in nominations that are focusing on the architecture of the particular property, whether it's a house or a historic district. Another useful tool are the Sanborn Fire Insurance maps. These are often available at your local library online, or if you're lucky they may have hard copies of the Sanborn maps for their communities. These allow you to track the building construction materials and uses of buildings and communities over spans of time. Local libraries are a great and underutilized source of information. Clipping files at your local library, almost every local library has a local history section and those may have many, many disparate resources that on their own may not be helpful, but collectively provide you a good historic snapshot of your community. Other resources are web-based sites like Wikipedia, which are sometimes poo-pooed, are actually very helpful in identifying historic facts about places, persons, or events, and they often lead you to other places on the web to find further information. So there's a vast world of information out there. The hard part is culling through what is not germane to the argument for why your property is important.