Lewis Wolpert has written a remarkably clear introduction to depression. Though his reasons for writing this are due to a personal episode of depression, he takes a step back and examines the possible causes and treatments available in a (mostly) cool and rational fashion. Generally, this is a very laudable aproach, though it can make relatively dry reading from time to time. (On the other hand, perhaps it is time that popular books which discuss Scientific and Medical topics should stop molly-coddling its readers) . For a more personal account, I would heartily recommend "Darkness Visible" by William Styron as a companion to this. Getting back to "Malignant Sadness", if there were a second edition, I think his chapter on the structure of the brain could do with some diagrams to help us visualise what's going on. Apart from these relatively minor quibbles, I found it an enormously informative book as an interested outsider. I would definitely recommend it for somebody who is living with, or taking care, of one who has depression.
One final note, though he tries to remain open minded about the various approaches to dealing with depression, it is pretty clear that he is very scornful of psychoanalysis (NOT cognitive therapy, which he is a big fan of). So if you're a big fan of Freud prepare to have your feathers ruffled.