The later work is philosophy rather than art history, though it is usually conducted through images. Coomaraswamy argued that Vedanta, Platonism, the scholastic Christianity of Aquinas and Eckhart, and Islamic metaphysics are not similar systems but one doctrine stated in different languages, and he wrote by setting Sanskrit, Greek, Latin and Arabic terms alongside one another to show that they translate the same distinctions. He called it the philosophia perennis, and the position is close enough to that of Rene Guenon for the two to be treated together as the founders of the Traditionalist current, though they worked separately and corresponded rather than collaborated.
The application he pressed hardest is to art. A traditional image is made to a canon, by a craftsman who is not expressing anything, in order to be used for contemplation. The originality of the maker is beside the point, and the modern category of aesthetics, the study of how a work makes one feel, obscures what the object is for. Why Exhibit Works of Art and The Transformation of Nature in Art set this out. It is a hostile account of the museum, written by a museum keeper.
The scholarship is uneven in the way of the perennialists generally: the philological detail is often excellent, and the overall claim is not the kind of thing evidence can settle. His work is read in art history, in Indology and in the Traditionalist current, and the three sets of readers rarely agree about which parts matter.