The phrase "study textbooks" refers to the act of reading and reviewing educational books to learn and understand a particular subject or topic. Full definition
In the 2016 - 17 school year, none of the social studies textbooks listed for use in the state's fourth grade classroom were published after 2005. (hechingerreport.org)
When I was not studying a textbook for an exam, I was reading an investment book. (thefinancialjourneyman.com)
Their work is a version of what educators call project - based learning, which means gaining knowledge by solving problems rather than by studying textbooks. (hechingerreport.org)