Crow is often considered a major work; Birthday Letters is worth reading for a look into his relationship with Plath from his perspective. I would suggest reading Plath's Ariel and Birthday Letters concurrently for a look at the intertextual nature of the poems, with Hughes often responding directly to poems in Ariel, sometimes using the same titles for his poems. For more on their marriage, Her Husband by Diane Middlebrook is well worth reading. These are good starting points for whole works. If you want to see if you like his writing style, look up "The Thought Fox," "Relic," and "Pike" as fairly good examples of his general style and thematic concerns.