You need to obtain permission any time your use of the work goes beyond the Fair Use guidelines set forth in 17 U.S.C. 107.
Fair use tends to fall on the side of nonprofit, educational use of factual, published materials.
One chapter of a multi-chapter book, one article from a journal, or one poem from a poetry collection tends to be good guidelines for determining how much of the work you can fairly use. Your use would probably be determined fair if it does not interfere with the potential market value of the work. A mistake that many people make is in writing the author for permission, but that author is not always the owner of a work's copyright. Write to the Permissions Department of the work's publisher. If the publisher does not own the copyright, the Permissions Department will be able to tell you who does.