Reporting Plagiarism
First be sure it is plagiarism by checking the textbook language with the original.Then contact the original author(s) to both alert them to this (possible) plagiarism and also to find out if they had given permission to the textbook author(s) to use their material. If that was the case, the textbook publisher should provide a correction, to note permission had been given and then use quotes.
Reporting Plagiarism by Someone at a Different Institution posted by kcunefare
The Plagiarized Sources and Site of Publication of Misappropriated Material posted by CarolineWhitbeck
Reporting Plagiarism posted by JohnAhearne