how IS a raven like a writing desk?

Because they both come with inky quills.

Because Poe wrote on both.

Because they can both produce a few notes, even though they are very flat.

Satisfied? Lewis Carroll is spinning in his grave.

Because the notes for which they are noted are not noted for being musical notes. (Puzzle maven Sam Loyd, 1914)

Because Poe wrote on both. (Loyd again)

Because there is a B in both and an N in neither. (Get it? Aldous Huxley, 1928)

Because it slopes with a flap. (Cyril Pearson, undated)

it's black as lead