Set as justified rectangles of text, often comprising a single, elaborate sentence on a page, the poems in Eamon Grennan’s new collection Plainchant (“these plain words—to be taken out at times of need”) appear at first so plainly not plain that a reader may wonder if the book’s title is ironic. Grennan, an Irish poet who taught for many years at Sarah Lawrence, remains beautifully “knacky”—artful, cunning—about the miraculous abundance of the world, but his intention here is as “bright and see-through and hard at once” as the window-shaped form of these poems. As with the liturgical music called “plainchant,” he seeks an unaccompanied line, a “hoist of song,” to express the inexpressible: the “pure worldliness” of two horses, the “single thing of wonder” that is a gannet’s flight, a memory of his mother’s “simple solid nearness,” “the day I’m passing through that’s passing right through me.”