Reference sources are invaluable for defining key concepts, providing brief overviews of a subject, and identifying major theories and research developments. They can provide you with the background you need to select and develop a topic of research, or help you explore internship or career opportunities.
These books are helpful for an overview of a topic. For a comprehensive list, consult the Nursing LibGuide.
Diagnostic and Statistical Manual of Mental Disorders DSM-5-TR (book) by 
Dictionary of American Nursing Biography by 
Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health by 
Pearson's Handbook of Laboratory and Diagnostic Tests by 
The Gale Encyclopedia of Nursing and Allied Health by 
Taber's Cyclopedic Medical Dictionary by The Merck manual of diagnosis and therapy, 18th ed.