Spiros suggested that the book refers to spiritual teachers (instructors). In Greece, when we refer to πνευματικός we mean the priest who is one's spiritual guide and to whom one confesses his or her sins and asks for absolution.
Well, yes and no. When you let's say begin a spiritual quest and start to be involved in all kinds of spiritual staff (e.g. yoga, tantra, all kinds of meditations, taro, prayers, angels, entities, etc.), you also can be called a spiritual person.