The author of the textbook should be well-educated and experienced in the subject of the book. In either the front of the textbook or the back, there should be a short "About the Author" or similar section. Read over this and make sure the author has the credentials necessary to write a textbook on the subject. Though credentials vary by subject matter, the author should have at least a master's degree in the subject, and should have experience teaching at a college or university. If he does not, he will not be able to provide as much useful information as a more learned author would.
Organization can make or break a textbook. Even if the author is well-qualified and educated and the information provided is in-depth and credible, if the information is disjointed and hard to follow, it loses its effectiveness. Good textbooks will have chapters that build upon each other as the reader continues through in terms of content. The chapters will be formatted in a uniform manner, and there will be an easily read table of contents, index and glossary. Chapter introductions and summaries should be succinct, and all pages should be numbered for easy navigation.
Textbooks are geared toward particular grade and learning levels. A textbook should be written to the audience it will be used to help teach. Differences among grade levels include vocabulary used, font size, number of pictures and graphics and the depth of the information. For example, an English literature textbook for second-graders would not feature a study on Shakespeare, but a high school or college textbook would.
Good textbooks will have a bibliography giving credit to all the sources used to write the book. This shows the reader that the information in the book comes from credible sources and that the author is not plagiarizing material or making up his own facts and figures. It backs up the credibility of the textbook as a whole.