
The translation of the Esh Kodesh is called Sacred Fire: Torah from the Years of Fury 1939-1942 and is translated by J. Heschy Worch. The English is quite readable and there are no explanatory notes or comments. There are many powerful messages contained in the translated divrei Torah, but as you would expect it is difficult to appreciate the Rebbe's chiddushim without knowing the sources that he is quoting.
The analysis of the Esh Kodesh is called The Holy Fire: The Teachings of Rabbi Kalonymus Kalman Shapira, the Rebbe of the Warsaw Ghetto by Nechemia Polen. He translates and analyzes portions of the divrei Torah and arranges them into categories.
I have used both books when teaching and I would recommend owning both of them if you can afford it. Both of them are available from Amazon.
Also, I know that R. Moshe Weinberger of the Aish Kodesh shul on Long Island has given shiurim on the Esh Kodesh and they can be purchased online.
Kol Tuv,
Reb Yaakov