Nefesh Ha-Chayim is a philosphical, mystical and kabbalistic work by Rav Chaim of Volozhin -- foremost disciple of the Vilna Gaon.
There’s a number of stories regarding the reason for creating the work. One of which I’ve heard is that the Gaon requested a non-chassidic response be created to Tanya by the Alter Rebbe of Chabad, and that’s Nefesh Ha-Chayim.
I once walked into a shiur (a Torah lesson) on Nefesh Ha-Chayim, and I didn’t realize for 20 minutes that it wasn’t Tanya being taught (meaning they cover many of the same topics and differ in very deep esoteric points only).
This new edition translates, for the first time, the kabbalistic sections of the sefer (previous English editions did not). The translator tells me his translation, “includes haskamote/divrei chizuk from two roshei yeshivot at Yeshivat Rabbeinu Yitzkhak Elchanan (YU-RIETS) and two local rabbanim in the NY/NJ area.”
This work, as well as Tanya, form the theological foundation of Judaism.
I have NOT reviewed this translation, but recommend everyone learn Tanya or Nefesh Ha-Chayim or such other foundational theological Jewish works. The further availability of such works in English is a blessing.