I haven't read that book, Alex, although I have read Brave New World. Mental illness could, in essence, be a function whereby the mind/brain gets overwhelmed and closes down, simply because it is not efficient enough to cope with what our perceived consciousness is trying to compute. The brain has billions if not trillions of neural pathways and connections, and I assume most are to do with keeping the body alive (thus to reproduce and pass on genetic infomation). I can give a weak example from my own experiences, which may have led to mental closedown:
I love music, mainly 'classical', and especially Beethoven, Mozart and JS Bach. When I was about 23 I was adamant that there should be some kind of mathematical equation that would provide computations that would lead me to being able to write the 'most tuneful melody ever'. Yet, my maths was only at GCSE level so I enrolled in a Open University maths course, level 1. But because I was impatient and got fed up with learning about how to calculate VAT on a packet of sausages I got depressed and my GP had to write to the OU to excuse me from the course. The thing was I was not bright enough (or patient) to correlate something that was ultimately non-linear and also subjective, which continued towards the bleakness of depression. You see I can compose music as well, so it seemed to me that there should be some rational way of producing music without waiting for my 'mind' to stumble across it. It is as if there is a block on my brain, ie it is not as efficient as Mozart's ,etc which I assume led to an increase in serotonin uptake.
Depression can also be founded by those around you. When you feel you have something amazing to tell someone, in my case my mum, and all they do is smile and humour you it just compounds the feelings of hopelessness. I am just a human being, a Homosapien. Only a tiny percentage of our species can tackle the myriad of quantum mechanics and chaos theory constructively without feeling suicidal. I had ideas above my station, but at least I tried, while all others where more worried about Man Utd or the Spice Girls.