What is "ghubar-e-khatir"

Ghubar-e-Khatir (Sallies of Mind), (Urdu: غبار خاطر) is one of the most important works of Maulana Abul Kalam Azad, written primarily during 1942 to 1946 when he was imprisoned in Ahmednagar Fort in Maharashtra by British Raj while he was in Bombay (now Mumbai) to preside over the meeting of All India Congress Working Committee.[14] The book is basically a collection of 24 letters he wrote addressing his close friend Maulana Habibur Rahman Khan Sherwani. These letters were never sent to him because there was no permission for that during the imprisonment and after the release in 1946, he gave all these letters to his friend Hakim Ajmal Khan who let it published for the first time in 1946.