As much as we would love to be able to copy entire textbooks for student access, it’s against the law as observed in Section 107 of Title 17, United States Code as amended in 1990 and 1992.
Fair Use states that copying "shall not substitute for the purchase of books, publishers’ reprints or periodicals,” and that “not more than one short poem, article, story, essay or two excerpts may be copied from the same author, nor more than three from the same collective work or periodical volume during one class term.”
If it’s a text book (or any book), no more than one chapter, or 10% of the book (whichever is less) may be copied and passed out to students.
This article is short and does a great job explaining it.
Also helpful are pp. 6-7 of Reproduction of Copyrighted Works by Educators and Librarians.