You need to be able to interpret what you read. Even textbooks represent an author's account or interpretation of theory or practice. E.g. textbook accounts of a socio-economic theory like Marxism tend to consist largely of an author's attempts either
to explain the theory from a particular perspective, or
to critique that theory in relation to other competing theories.
Part of the skill of reading into a writer's perspective - interpreting their position - lies in developing an awareness of the role language plays.