You won't find it in any science book, it isn't empirical in nature. Philosophy won't infallibly take you there. But I believe it is an ontological primitive, like matter which is just as basic though perhaps not quite. Lots of physicists have commented on this including Niels Bohr and Max Planck. A few quotes all taken from Iain McGilchrist's book The Matter With Things:
Mathematician/physicist von Neumann: ‘..it is inherently entirely correct that the measurement or the related process of the subjective perception is a new entity relative to the physical environment and is not reducible to the latter. Indeed, subjective perception leads us into the intellectual inner life of the individual, which is extra-observational by its very nature.’
Adam Frank, Professor of Astronomy at the University of Rochester, New York: 'we must entertain the ‘radical possibility that some rudimentary form of consciousness must be added to the list of things, such as mass or electric charge, that the world is built of.’
Regarding our reluctance to even entertain the notion that consciousness may have a rudimentary form of existence which predates brains, philosopher/psychiatrist Iain McGilchrist has written:
..a form of anthropomorphism operates in reverse: ‘doesn’t do what humans do with their consciousness, so can’t have it’. Sometimes I am asked, ‘surely you can’t think a mountain has awareness?’ I feel like replying, ‘but how would you expect a mountain to behave if it did have awareness? Mow the lawn, drink a beer and go to Sainsbury’s?’ The idea that consciousness is an ontological primary looks much less bizarre in any other culture than our own. We don’t have the luxury of adopting a common-sense attitude here, because our alternatives are (a) either consciousness does not exist at all (see above[regarding Dennett]), or (b) it was there all along, in everything. Which is the more absurd?