The phrase "textbook material" means something that is perfect or ideal to be included as an example or explanation in a textbook. It refers to something that is standard, typical, or worthy of being taught or studied. Full definition
Today, master teachers collect filing cabinets of excellent paper activities to supplement textbook materials. (teacherease.com)
The projects will take on various tasks, such as recruiting high - quality teachers, providing professional development to current teachers, improving textbook material, and encouraging students to take challenging coursework. (edweek.org)
For the second, strong backup, it seems to me that an important element would be the existence of adequate textbook material to which a teacher can point if faced with opposition. (realclimate.org)