Characters and Viewpoint by Orson Scott Card
This book is one of the most frequently cited texts in workshops and writing panels. The MICE quotient, Cinematic Third Person and many other methods writers use today were first introduced by Card in this book. While I personally don’t ascribe to all the methods, I love recommending it because it’s succinct and shares a lot of principles that are useful for scriptwriters.
Kicking in the Wall by Barbara Abercrombie
Books of writing prompts are usually not good for much else other than warms ups and pure technical training. This one is special. The exercises are well thought out, and can be applied to your character’s experiences or your own. Either way, I find that they draw out fresh conflicts and details. They compel my characters to move.
Alone with All that Could Happen by David Jauss
This book has the most eloquent explanations of conflict and point of view that I’ve come across. It’s directed more towards novelists than screenwriters, though. A good alternative for this if you are just just focused on screenwriting is Judith Weston’s The Film Director’s Intuition, which has a part on rehearsal techniques that can also provide a nuanced understanding of character perspective.
45 Master Characters by Victoria Schmidtt
For the most part, this book is exactly what it sounds like. It categorizes and describes character archetypes. Good for both writers and academics, but the real meat is in the back. She breaks down methods of change for characters (character arcs) and presents dichotomous solutions to well known character structures. (For example, the feminized version of the hero’s journey that we witness in The Wizard of Oz)
Anything by Carl Jung
This guy breaks down symbols and the way that pervasive narratives represent and shape our inner conflicts as human beings. If you want to write human characters, this will fascinate you. Start with his theory of the anima/animus; it will give you a deeper understanding of what characters are to us and to each other.
I hope that those resources prove useful for you. If you’re looking for good examples of stories that characterize people sublimely and are easy to learn from, check out Inu-Yasha, Breaking Bad, the novels of Brandon Sanderson and anything by Jane Yolen. It’s also useful to examine multiple retellings of a classic story and witness how the characterization has changed across versions, as well as the traits that are preserved.