There are two ways to talk about the new Letters of Ted Hughes (Farrar, Straus & Giroux, $45), edited by Christopher Reid. The first is to approach Hughes’s correspondence as an illuminating aesthetic record, the clearest insight we’re likely to get into the mind of a poet viewed by some critics as one of the major writers of the 20th century. The second way is to discuss, well, “It.” “It,” of course, is what Hughes called “the Fantasia,” the swirling, ­decades-long hoo-ha brought about by his relationship with Sylvia Plath: their brief, difficult marriage; their separation due to Hughes’s affair with Assia Wevill; and Plath’s suicide shortly thereafter. “It” ultimately involved a series of bitter clashes over Plath’s legacy, the occasional illicit removal of the surname “Hughes” from her tombstone (by aggrieved “Bell Jar” fans), a series of disputed biographies, at least one lawsuit, endless critical appraisals, re­appraisals and re-­reappraisals, a lame song by Ryan Adams (“I wish I had a Sylvia Plath,” Adams croons, apparently unaware that they don’t come in six-packs) and the inevitable film featuring Gwyneth Paltrow flopping around with Daniel Craig. “It” is a big deal.