A textbook is a manual used to teach a subject. Textbooks are produced based on educational demands and provide instructional material. They serve several functions like being a language source, learning support, and motivation. Textbooks are evaluated based on criteria like meeting learner needs and program objectives. The process of developing materials involves preparation, representation, selection, and adaptation for learners. Characteristics of good materials include achieving impact, relevance, feedback, and exposing learners to authentic language use.