To add from psychologists, psychiatrists, and counselors, there are many great theorists and collective books. Freud would be the classic, as a psychoanalyst. I know from other theories that Carl Rogers has On Becoming a Person and the Carl Rogers Reader regarding Person-Centered Theory(I have read selections from both but have not gotten to read them as a whole yet!); Yalom writes on his position as an existential therapist (though he did not create the theory); and I have heard Albert Ellis wrote prolifically regarding his Rational Emotive Behavior Theory. Honestly there are many theories in the helping professions, so it depends on what direction you want to look: psychoanalytical, humanistic, cognitive? Even just researching regarding these schools of thought could be good information! Good luck!