Read the "Introduction" or "Preface" to understand the purpose of the textbook. This section will also provide you with information about whom the textbook intends to target. If there are no direct statements to this effect, analyze its relevance by observing the terminology, examples and range of information the book provides.
Check if the book provides the author's profile and read this thoroughly to gauge the author's expertise in the chosen field. Look out for information on other publications the author has to his credit in the same area. Search research databases relevant to the subject to see how many research papers and academic publications cite this textbook in their bibliographies.
Study the "Table of Contents" to find out the depth of coverage. Analyze if the textbook is just an overview of work in that field or if it is a comprehensive analysis of various topics of significance. Examine the "Contents" section to understand how the book is organized. Check for logical and linear flow of information, which makes for easy understanding of the subject.
Check the "Bibliography" for length, selectivity and citation style. Determine if the references cited are primary sources like journal articles or secondary sources like encyclopedias. Identify a few of the references and examine these to verify if they are based on facts or opinions. References based on facts are generally more reliable because opinions can vary based on how the facts are interpreted. To understand the relevance of the references to the topic of the textbook, check that the references are up-to-date. Outdated references can mean no real value-addition in the case of subjects like medicine and information technology, where changes occur rapidly.
Evaluate how useful the textbook is to your purpose by scanning various chapters. Look out for statistical data, case studies, anecdotes, charts, photographs and graphs that can make the topics easy to understand. Observe the general line the author takes to support or refute a concept or practice and check if this is relevant to your field of work.