The book is a compendium of his talks explaining Buddhism given while opening the Karma Dzong meditation center in Boulder, Colorado.
[1] Conventionally, it is used to describe capitalist and spiritual narcissism, commercial efforts such as "new age" bookstores and wealthy lecturers on spirituality; it might also mean the attempt to build up a list of credentials or accumulate teachings in order to present oneself as a more realized or holy person.
In Trungpa's presentation, spiritual materialism can fall into three categories — what he calls the three "Lords of Materialism" (Tibetan: lalo literally "barbarian") — in which a form of materialism is misunderstood as bringing long-term happiness but instead brings only short-term entertainment followed by long-term suffering: The underlying source of these three approaches to finding happiness is based, according to Trungpa, on the mistaken notion that one's ego is inherently existent and a valid point of view.
The message in summary is, "Don't try to reinforce your ego through material things, belief systems like religion, or certain emotional states of mind."
In his view, the point of religion is to show you that your ego doesn't really exist inherently.