Originally published in 1924, this treatise contains some dated concepts and views. However, if you allow for these innocuous positions then this is an intersting and informative analysis of how behaviourism, as Watson explains it, relates to the psychology and psychoanalysis of the time.
In summary, as human beings, what we learn and how we behave is the result of the totality of the stimulus applied to us, as an organism, from birth until we mature. Covering topics such as the human body, emotions, words, language and thinking; Watson provides observational conculsions from formal experiments, to anchor behaviourism as a modern science.