Horn, who died in 2005, was a jazz singer and pianist whose style's watchword is "intimacy." Her vocal manner is akin to Miles Davis' trumpet style: not technically flashy, but definitively setting the song's mood. Indeed, it was Davis who discovered Horn in 1960 and who became an admirer despite his well-known dislike of vocalists generally. Horn was never a big record-seller, but she was well-respected by her peers.

To the list of vocalists Last.fm says she resembles, I'd add Diana Krall: both have strikingly similar ways of phrasing, and Krall's voice's timbre is just a bit lighter but otherwise is very similar to Horn's.

Shirley Horn is also known for her distinctly slow tempos and complete understanding of 'space' in her singing and piano playing.

Here is the Wikipedia entry for Horn.