Field Work is Seamus Heaney's fifth book of poems, and his first to be published in this country. His strong and beautiful verse already has a following on American campuses, and this fine new book will undoubtedly extend the range of his influence. His Ireland is an ""island. . . full of comfortless voices""; he lived in Belfast for many years, as the elegies for the war dead (""Casualty,"" ""A Postcard from North Antrim,"" etc.) and other troubled poems attest. ""Ugolino,"" the translation of a passage from The Divine Comedy, also concerns war and vengeance, and injustice: ""For the sins/ of Ugolino, who betrayed your forts,/ Should never have been visited on his sons."" The delicate ""Elegy"" is a memory of Robert Lowell's last days. But Heaney's Ireland is also quotidian: ""She came every morning to draw water/ Like an old bat staggering up the field:/ The pump's whooping cough, the bucket's clatter/ And slow diminuendo as it filled,/ Announced her."" Ten ""Glanmore Sonnets,"" which occupy a central place in the book, commemorate an easing in the poet's life, a time when ""Vowels ploughed into other: opened ground""; and this is the ""field work"" of the title poem as well. Looking at an ugly badger, Heaney asks: ""How perilous is it to choose/ not to love the life we're shown?"" Waist-deep in language, he does. A fine introduction to a staunch, resonant spirit.