A harp is a multi-string musical instrument. It is popular in Westeros amongst all of society, and is often used to accompany a song. The high harp is mentioned as making soft sweet sounds. A musician who plays the harp is called a harpist or harper.
A Clash of Kings
In order to plant the notion of Ser Loras Tyrell taking the white Littlefinger slips a few silvers to Lord Tyrell's army of singers to sing of Ryam Redwyne, Serwyn of the Mirror Shield, and Prince Aemon the Dragonknight. He tells Sansa Stark that a harp can be as dangerous as a sword in the right hands.
Prince Rhaegar Targaryen appears in one of Daenerys Targaryen's visions in the House of the Undying. At the end of the vision he goes to the window seat, picks up a harp, and runs his fingers lightly over its silver strings. Sweet sadness fills the room as the vision fades, and only the music lingers behind.
A Storm of Swords
Tom of Sevenstreams tells his companions that Lady Lysa Arryn once sent him up the high road, where the Moon Brothers took his gold, horse, and clothes. He walked up to the Gates of the Moon with nothing but his harp. There the guards made him sing two songs before letting him pass.
A Feast for Crows
- Dareon, who plays a woodharp
- Domeric Bolton, who played the high harp.
- Guyard Morrigen 
- Hamish the Harper
- Leonette Fossoway, who plays the high harp
- Marillion, who plays a woodharp.
- Michael Manwoody
- Queen Naerys Targaryen
- Lord Pearse Caron
- Prince Rhaegar Targaryen, who played a high harp with silver strings
- Rylona Rhee
- Sansa Stark, who was taught to play the high harp by Leonette Fossoway
- Lord Simon Dondarrion, who played the high harp
- Symon Silver Tongue, who plays a twelve-stringed woodharp.
- Tom of Sevenstreams, who plays a woodharp.
- Unnamed brother on the Quiet Isle, who plays the high harp
- Rymund the Rhymer
When you heard him play his high harp with the silver strings and sing of twilights and tears and the death of kings, you could not but feel that he was singing of himself and those he loved.
—thoughts of Cersei Lannister
At the welcoming feast, the prince had taken up his silver-stringed harp and played for them. A song of love and doom, Jon Connington recalled, and every woman in the hall was weeping when he put down the harp.—thoughts of Jon Connington
A harp can be as dangerous as a sword, in the right hands.
- A Feast for Crows, Chapter 31, Brienne VI.
- A Storm of Swords, Chapter 61, Sansa V.
- A Clash of Kings, Chapter 48, Daenerys IV.
- A Storm of Swords, Chapter 6, Sansa I.
- A Storm of Swords, Chapter 43, Arya VIII.
- A Feast for Crows, Chapter 41, Alayne II.
- A Feast for Crows, Chapter 15, Samwell II.
- A Dance with Dragons, Chapter 32, Reek III.
- A Clash of Kings, Chapter 22, Catelyn II.
- A Storm of Swords, Chapter 60, Tyrion VIII.
- A Game of Thrones, Chapter 28, Catelyn V.
- The World of Ice & Fire, The Sisters of Baelor I.
- The World of Ice & Fire, The Targaryen Kings: Viserys II.
- The Hedge Knight.
- A Storm of Swords, Chapter 42, Daenerys IV.
- A Feast for Crows, Chapter 24, Cersei V.
- A Dance with Dragons, Chapter 11, Daenerys II.
- Fire & Blood, Birth, Death, and Betrayal Under King Jaehaerys I.
- A Storm of Swords, Chapter 32, Tyrion IV.
- A Storm of Swords, Chapter 22, Arya IV.
- A Clash of Kings, Chapter 45, Catelyn VI.
- A Feast for Crows, Chapter 10, Sansa I.
- A Dance with Dragons, Chapter 61, The Griffin Reborn.