Before outlining, the student must understand the general purpose of the textbook, the audience which it addresses and the list of contents laid out to accomplish its purpose.
A introductory note or preface which elaborates on "how to use this book" would normally suffice to understand the authors' goal as well as the aim population the book was addressed to. One or two pages further, an index or list of contents should provide the outliner with a clear picture of all the subtitles expected to help achieve the major educational purpose of the book.
After reviewing these important sections in a book, one can then proceed to outline it.