To be suitable to use in classroom, reading material must fulfill specific requirements, first at all, it should be relevant, this means related to learner's goal and interests, second, the level of difficulty should be the appropriate, this factor it's determined by vocabulary, students must know the most of the words in the text to understand it, experience in teaching can help to evaluate this or teacher can check the words in the reading material against reference lists to find out their level of frequency and difficulty, 3,000-5,000 words are required to understand advanced level or authentic material. A challenging text, can still being used, but teacher should be aware of the required English level and the kind of reading tasks needed.
The other aspect involved in level of difficulty is the sentence length, longer the sentences, the more complex structures, affecting how easy a text is to read and understand. The fourth factor to consider is the length of the text, a text is long if it has over 400 words, class time won't be enough to read it in classroom, will be necessary get students interested in the topic and pre-teach vocabulary, and ask them to read it at home, the alternative is re-write and shortening the text, to fit with the class time available. The decision of having a textbook, carries with some advantages and disadvantages. The pros are that, they're written for a specific level, vocabulary and sentence structures cover book's scope, tasks are already written, saving preparation time, all materials are in the