What is the ruling on studying body language and interpreting its gestures based on scientific studies and books, and is it permissible in Islamic law?

The science of body language is not inherently forbidden. It does not fall under sorcery or charlatanism. Rather, it is a way of dealing with apparent causes (body movements) and attempting to understand and analyze them. It is similar to "physical physiognomy," a field in which doctors and others classified and inferred character from physical creation, due to the connection between them.

The origin of this physiognomy is that the moderation of physical creation and form leads to the moderation of temperament and spirit, from which arises the moderation of character and actions. Physiognomy has two causes: the quality of the observer's intellect and the appearance of signs in the observed. When these two combine, error is rare.