5. Colm Tóibín’s On Elizabeth Bishop, an exquisite study of Bishop’s poetry, including her great New Yorker poem, “At the Fishhouses,” which Tóibín ingeniously connects with James Joyce’s “The Dead.” (See my post here.)
Correction: I see now that Michael Hofmann's Where Have You Been? is not a 2015 book. It was published in 2014. But I read it this year, and so I'm going to keep it on my 2015 list.