Reference works like dictionaries and encyclopedias, some of which are highly specialized, are useful for:
When you come across useful words, concepts, keywords and names related to your topic, make a note of them to use in further searching.
Reference sources can help you:
APA PsycBooks -- includes the APA Handbook of Clinical Psychology; also includes hundreds of APA-published books
Credo Reference -- "Background info to start your research" from top university presses and other scholarly publishers.
Individual reference books (find more by searching in CLIO and limiting according to "Subject/Genre")