Q. How much of a textbook am I allowed to copy and give to my students?

The law on copyright includes a provision called "Fair Use" which allows for certain uses of material in copyright. You can read more about Fair Use on my Copyright Toolkit under Fair Use. There are no hard and fast rules that state how much you can legally use in this way, but there are several tools to help you think through your Fair Use case. Our favorite is "Thinking Through Fair Use" from the University of Minnesota Libraries, which allows you to put in several factors like the purpose, amount, and nature of your use. 

Generally, the rules do not differ between print and online use this way. For a textbook, usually one or two chapters (depending on the length of the book, of course - the use needs to be proportionally small compared to the length of the book) is acceptable under Fair Use. If you need to use more than that, there are a few options available to you. One is to put a copy on course reserve. Another is to ask the library to purchase an ebook version, if it is available, and put that on your course's e-reserves. Here is some more information on reserves, and here is the request form for purchasing a book.