Rav Aharon wrote the following:
1) שערי היחוד ואמונה – first published in תק”פ, approximately seven years after the death of the Baal HaTanya. This sefer discusses many of the basics of the הקדמות that you need to properly understand the kaballah behind the Baal HaTanya, and it can also serve as an introduction to עץ חיים for those more kaballah oriented. It was recently reprinted (just this past year) after being out of print for a long time. The reprinted version is truly a pleasure to learn from.
2) The continuation to that sefer is called שערי עבודה. It takes the hakdamos from the שערי היחוד ואמונה and brings them into the realm of actual avodah. Having a basic understanding of Tanya, in addition to the first sefer, is important in this sefer. It has not yet been reprinted, but I believe that it will be soon IYH.
3) He has what was originally a 3-volume sefer called עבודת הלוי which are maamarim על התורה. Until recently it has been very hard to come by, but it is in the process of being republished. My information may be a little out of date, but I believe they have published two of what will ultimately be a four volume set.