Loss of interest is arguably the most significant symptom of severe depression yet it does not seem to attract much investigation. It would seem there is only one book chapter solely devoted to that topic:
Loss of interest. Klinger, Eric
Costello, Charles G. (Ed). (1993). Symptoms of depression., (pp. 43-62). Oxford, England: John Wiley & Sons, x, 326 pp.
Anyone know of any other study on this important symptom?