I am writing a book. It will be post-apocalyptic, and the apocalypse I'm thinking of will involve people being able to read each other's mind. Can any of you give me a slightly plausible reason that this would happen / be possible?
If you don't need the mind readers to actually know the exact "words" of the person's thoughts, you could have people who are extremely well versed in "reading" another person's facial expressions, body posture, tone of voice etc. It is a fact that those outward behaviors reflect your internal state, and in fact we all read these signs with more or less accuracy, but there are a few people (and maybe you can train that) who are better than others to deduce how you feel and what you might be thinking from these. (see "cold reading")
If you need actual mind readers, we are currently arriving at a point where we begin to be able to interpret fMRI images. There are some experiments where researchers were able to tell which images a test subject was looking at or which body parts they were moving from looking at their brain activity alone. I'm sure that our understanding of what goes on in the brain will progress and the technology to measure brain activity will get smaller and more exact, so you could have people use a device that reads minds. It is quite likely that such a device will exist in the not too far future.
If, finally, you want real telepathy (which, just to cause no misunderstanding, cognitive scientists believe does not exist) you might base it on:
an organ in human beings sensitive to magnetic or electric fields, sensing (like EEG) the brain states of another human being; there are animals who can sense electricity or magnetism, but the problem with this is that magnetism surrounding us is stronger than that coming out of the brain and will make it impossible to "feel" a brain from farther than a few centimetres away
this organ might be in the skin (actually not one organ but many receptors) so the reader could cover the head of the person to be read with his hands, much like an EEG cap
the reader would need to be trained with people telling them what they think while he "feels" their brain, so that he can learn which "sensations" in his hands correspond to which thoughts and cognitions
maybe its possible to clone human beings with specific animal genes so that humans grow electrically or magnetically sensitive receptors, e.g. in their hands (research animals with electric and magnetic senses and how their receptors work)
I'm just trying to extrapolate (my understanding of) current research, here. I cannot foresee future discoveries either technological or psychological, so I'm keeping this within the realm of the possible as much as seems plausible to me. Again: We don't think telepathy in the traditional sense (concentrating your mental powers and knowing what someone thinks) exists. If you want that, I don't see how it could be possible.
There seems to be some evidence of magnetoperception in humans:
It is easy to imagine that this sensitivity to the earth's magnetic field might develop to discern even the much weaker magnetic fields of the brain. After all, the other senses show a huge variety between species, too. For example, the silk moth can detect single molecules of a female's pheromones. The current sensory ability of humans is not a fixed limit for how sensitive a sense can be. If the magnetism of the brain can be measured by an artificial detector, then it can potentially be measured by a biological detector.
My idea was similar to what "what" said; however, instead of the reader needing to be trained to the thoughts of other people and/or having to clone humans with these genes, the OP could have it to where the post-apocalyptic world in which the book is set in features humans that have evolved into what they are now, with having these receptors in their skin included. Similar to your statement about body language detection, some people's electroencephalographic receptors might be better at detection, much like our eyes and other organs.
As @what wrote, the state of our knowledge on Mind reading is low but it is an on-going research. The current research uses EEG and MRI techniques to retro-engineer (read and decipher) our thoughts and actions: see the Introduction To Modern Brain-Computer Interface Design linked to the EEGLAB software, or the Brain computer interface wikipedia page (http://en.wikipedia.org/wiki/Brain–computer_interface).
The most straightforward way to get to the apocalypse you described would be, in my opinion, the desire of human beings to enhance their cognitive abilities (called Transhumanism, see Could neuroscientific knowledge and techiques be used to optimise peoples' education and learning?). This would be realised by having some stimulation electrodes in the brain (we actually do that for Parkinson). However, these electrodes could be hacked and be used as EEG readers. Smartphones would be an ideal candidate for such a hack (see how smartphones can already be coupled with EEG, http://milab.imm.dtu.dk/eeg).