Reference resources help you choose, explore and develop your topic! Reference sources such as encyclopedias and dictionaries contain overviews and background information. They sometimes contain statistics, definitions and lists of resources for further study. When you have a better understanding of your topic, it will be easier to come up with the search terms you'll use to find the books and articles you need.
Wikipedia is a reference source because it is an encyclopedia. You can use it to discover ideas and links to additional resources -- but never cite it as a source in your papers! This rule exists because Wikipedia entries can be edited at any time, by anyone, and so its accuracy can never be relied upon at any given moment.
A Dictionary of Architecture and Landscape Architecture by 
Dictionary of Architecture and Construction by 
Encyclopedia of 20th-Century Architecture by 
A Global History of Architecture by