I do not consider the Big Book a textbook. In order for it to be considered a textbook, it would need to be based on empirical evidence. The Big Book is not assigned to medical student studying addiction/alcoholism. At best, I'd call it a guide, or practical advice. There are many such guides when it comes to addiction. There are several other versions of 12 step programs, for example, and also many books on recovery that do not include taking steps or adopting supernatural beliefs in order to recover.