A common feature of most textbooks is discussion questions at the end of every chapter that students can test their knowledge on. Alternatively, many teachers will assign the questions at the end of a chapter as homework or will use those questions as a basis for tests. The purpose of reading a textbook is the acquisition of the knowledge it contains, and discussion questions are a good way to test that knowledge acquisition.
Many textbooks also come with an index at the end of the textbook. Indexes are invaluable for those who wish to take a nonlinear approach to learning via a textbook, or for those students who wish to go back and reread what a textbook has to say on a certain subject. Interestingly, some schools are moving to digital textbooks, which can be searched much like a text document and make an index unnecessary.
Many textbooks also contain references, which serve several purposes. The first is to demonstrate where the knowledge came from. The second and probably more important purpose is to give students and educators a chance to further explore a subject area. A student looking to satiate her interest in a given subject has somewhere to go, and teachers looking to explore a certain subject in more depth in the classroom can use those references to explore potential material for that purpose.
Illustrations and Examples
The primary purpose of a textbook is to transfer knowledge, and with that in mind most textbooks endeavor to make that transfer of knowledge as simple as possible. Most often go over important subjects several different times, often by using examples and illustrations to further explain a subject. It is also well known that some people learn better with visuals, and textbook creators no doubt take this into consideration.