The Diagnostic and Statistical Manual is the standard reference for describing mental health disorders; the 5th edition is the most current published. The DSM-5 is located in the library's reference collection or you can access it online.
Do you have a basic idea what topic you want to research but have no idea where to go with it? This database of reference books can give you a broad overview of the topic that may help you narrow down your research topic or give you a direction to head with it.