The fatal union of two poetic titans, the deadly foreshadowing of Sylvia Plath’s posthumously published “Ariel” poems, her appropriation by the feminist movement — all have combined to make Plath an irresistible (if perpetually resistant) biographical subject. Even Ted Hughes, the estranged husband and British poet laureate who controlled her estate, censored her work, and resisted interviews about her, managed to have his say before he died in the moving verses of “Birthday Letters” (1998).

And the deluge continues. With the 50th anniversary of her suicide approaching, two more biographers have plunged into the melee, assessing both Plath’s prodigious gifts and her dysfunctions.