Apart from being one of the true heroes of the Warsaw Ghetto and inspiring countless others during the darkest days of the Holocaust, the Aish Kodesh wrote books which are understandable, practical, inspiring and relevant. There are English translations of several of his books, but if you can read a little bit of Hebrew you might want to try in the original.
My 12 year old son is learning Chovas HaTalmidim at school this year as his mussar sefer. It is a shame really, because people come to think of it as a kids' book. It is even more relevant for adults.
Here are the bios from the web:
Kalonymus Kalman Shapira was born in Grodzisk, Poland to his father, the Imrei Elimelech of Grodzhisk. Named after his maternal great-grandfather, the renowned Maor VaShemesh, he was a scion of a distinguished family, which included Rabbi Elimelech of Lizhensk, the Chozeh of Lublin and the Maggid of Kozhnitz.
At the age of three, he was orphaned by the death of his father. In 1905 he married Rachel Chaya Miriam, daughter of his nephew Grand Rabbi Yerachmiel Moshe of Kozhnitz. She helped him prepare his lectures and books, even adding pertinent insights of her own.
In 1909 he was appointed rabbi of Piaseczno, near Warsaw, and subsequently attracted many chasidim. He was deeply focused on the education of children and young men, establishing the yeshiva Da'as Moshe in 1923.
Rabbi Shapira's only son and his son's wife were killed during the Nazi aerial bombing of Warsaw in September, 1939. After the Nazi invasion of Poland, Rabbi Shapira was interned with a few of his chasidim in the Warsaw Ghetto, where he ran a secret synagogue. He wrote a book while in the camp and told some of his students where he buried it. Later The students went back and recovered his inspirational speeches of this period from the ruins of the ghetto and published under the title Aish Kodesh.
After the Warsaw Ghetto Uprising was crushed in 1943, Rabbi Shapira was taken to the Trevaniki work camp near Lublin. Prisoners who were completely worked out by exhaustion and starvation were removed and sent to the Treblinka extermination camp. Rabbi Shapira spent his last few months in Treblinka, where he was murdered by the Nazis in 1943.
* Chovas HaTalmidim (The Students' Obligation) - a collection of essays aimed at teenagers which has become a standard textbook in yeshivos.
* Hachshoras HaAvreichim (Preparation of Young Men) - a work written for young married men.
* Mevo haSheorim
* Bnei Machshava Tova
* Derech HaMelech (The Way of the King) - Torah discourses spoken on shabbats and festivals (1889-1943)
* Eish Kodesh (Holy Fire) - his inspirational speeches given during the Holocaust period.
* Conscious Community: A Guide to Inner Work - originally published in Hebrew, this English translation (translated by Andrea Cohen-Kiener) is based on the manuscrpits recovered from the rubble in the Warsaw ghetto. The book is a guide to attaining spirituality despite adversity and physical needs.
This is from Shema Yisrael
R. KALONYMUS KALMAN SHAPIRA, REBBE OF PIASESZNE
(note: the first part of this essay was incorrectly attributed to Prof. Herszel Klepfisz)
by R. Asher Katzman
In R. Shapira's work, Chovas Hatalmidim, he manifested his greatness in writing. He had a gifted pen, and was noted as a deep thinker and writer. His words make an impression on the inner core of the soul and heart.
In Warsaw in 5692, he published his Chovas Hatalmidim. It has been said that "the letters of this book burn with holiness." In this book, he penetrates the hearts of students, unveils their spiritual strength, and brings them to the Torah and service of God by educating them with a supernal love, with holy words and deep thoughts from the revealed Torah of God, and the broad, deep world of mysticism and kabbalah, expressing the character of his soul and mind.
R. Shapira commented on this work to the Gritzer rabbi, R. Eliyahu Lifschitz, "If I didn't know that I was the author, I would never have believed that I have the ability to write such a work ... It was only with strength that came from God..." This work created legions of yeshiva students who placed his words, which were like hot coals and flames, in their hearts, with the greatest passion and love for Torah and Hasidism. Even in the time of the great and terrible sacrifice of Polish Jewry, in the terrible camps of the death camps, beneath the shadows of gas chambers and burning crematorium fires, his words warmed them with great faith in God, so that they could accept martyrdom with joy.
The book is divided into thirteen chapters, corresponding to the thirteen principles of faith and the thirteen rules of interpreting the Torah, followed by three important essays on how to learn and ponder the teachings of Hasidism; how to understand and elevate oneself in Torah, prayer and song to God; and the holy exaltation of the manifestation of God on the holidays and the holy Sabbath.
When he published this book, R. Shapira was barely 42 years old, but he was already considered an extraordinary man, one of the leaders of his generation, a tzaddik and a holy man.
May His Soul Be Bound in the Bonds of Eternal Life and May G-d Avenge his Blood.