The Mood Cure by Julia Ross really saved my bacon after I ran a marathon in 2004. I thought I just had the after-a-big-event-blues, but it turns out that it is not uncommon for people to feel down after a marathon because people use up their mood producing amino acids. (It's an even worse scene for ultra-marathoners.) That's right, sometimes you don't have to dig deep into your psyche to find out what is wrong with you, you just need to pop a supplement, and that's ok.
The Mood Cure explains how you can spot a "false mood" and what nutritional deficiency that might be caused by. There are people who have structural damage to their brains which can cause personality changes, but for most people false moods or moods of any kind are caused by chemicals in the brain. You can also affect your mood with your thoughts, and with your environment (like getting enough sun,) and your behavior (like getting enough sleep,) but they affect your mood via chemical changes in your brain and "surprisingly brainlike areas of your heart and gut." The idea in the Mood Cure is that if you are severely deficient in a nutrient, you cannot produce the necessary chemicals to keep your mood steady even if you are thinking good thoughts etc. Julia Ross recommends supplements. For people who are deficient in certain nutrients because of diet, they will only need to take supplements for awhile while they are getting their diet back on track. For some people who have trouble creating certain chemicals, they may need to keep taking certain supplements.
This is one of my top 5 recommended books because it can take your experience of life from very miserable to just fine in as short a time as a week with some very simple changes in your diet and some fairly cheap supplements that you can find at any health store.