Stephen Ratcliffe's [where late the sweet] BIRDS SANG (O Books, 1989), an erasure of Shakespeare's Sonnets. (See also Jen Bervin's Nets, and K. Silem Mohammad's "Sonnograms" -- more here, here -- of course.) According to the publisher, the book contains "A rift of poems which pluck a few words from Shakespeare's sonnets and on that absent space create their own space and shape."

As Andy Frazee explains here, in Ratcliffe's modification, "All traces of Shakespearean voice or Elizabethan English have been eroded away.... Without knowing this is a treatment...of the Sonnets, we would probably not associate these texts in any way with their Shakespearean source...."

And, excellent articles on Shakespeare erasures by Vincent Broqua in Jacket2 here and in Transatlantica here.