There are books about Body language. This is one. The Dewey Decimal call number is below. There might be others in your library.
Body language : the secret language of body gestures and postures that reveal what we really think and mean
by Boyes, Carolyn.
Year/Format: 2005, Book, 191 p. :
Subjects: Body language.
302.222 BOY
2949488
Mastermind : how to think like Sherlock Holmes
by Konnikova, Maria.
Year/Format: 2013, Book, x, 273 p. ;
Subjects: Doyle, Arthur Conan, 1859-1930--Characters--Sherlock Holmes.
Holmes, Sherlock (Fictitious character)--Knowledge--Logic.
Holmes, Sherlock (Fictitious character)--Knowledge--Reasoning.
Logic.
Reasoning.
153.4 KON
I think that the Sherlock Holmes stories themselves should give you insight. Note that Holmes observes facial features, hands, clothing, the ways they move, what they say and how they say it. He is always looking for details. What is out of place. What is there that should not be and what is missing that should be there. Note how he deduces Henry Baker's appearance and life from his hat in "The Blue Carbuncle" , Dr. Mortimer in the first chapter of "The Hound of the Baskervilles", "Jabez Wilson in "The Red-Headed League" and how he and Mycroft observe the man beneath their window in "The Greek Interpreter" Note also that Holmes built up an encyclopedic knowledge of crime, criminals, soils, tobacco ashes, newpaper type-fonts. The man had an insatiable curiousity and was studying when he wasn't taking cocaine because he was bored & sorry for himself or when he was investigating a crime.
I also think books on creative writing - about creating a character - would also be of use.