Hi 'helenora'!
Thinking into the material takes in a complex idea of thinking by idea and logic.
You can ask some questions for instance. Why does the author say that or this. What logic or ideas are they depending upon to give this position or put forth this opinion.
Especially important in complex works such as mathematics, science or philosophy. What is the author assuming I know or agree with here?
You think out a path similar or better than author especially if you are compounding several books on the same topic or subject.
I ask if this line of thinking is useful, practical or can be applied to goodly 'effect'. Who else uses it?
Is this a theory? A fiction or some meandering or rehashing of someone else's work.
To think into something is like taking a walk through the same terrain as the author might have.
Kind Regards!