Nehora.com (http://www.nehora.com) has put together a great page on Aharon Ha-Kohain with some great facts...
Born: Egypt, 1406 BCE.
Died: Hor HaHar, 1283 BCE.
Prophet, brother of Moshe Rabeinu (Moses) and Miriam the Prophetess, the first Kohen Gadol (High Priest). The chariot for the Sephirah of Hod.
The Ari says that Aharon HaKohen, was Haran (brother of Avraham Avinu) in his previous life. Haran (same letters as Aharon, without the Alef) had to correct an element of idol worship committed by Adam, but he failed. As Aharon he should have also sacrificed his life, like Chur ben Miriam did, in order to stop the sin of the "Golden Calf" from taking place, in order to do a correction concerning idol worship. Mistakenly, he thought that Chur's death accomplished that. (Sha'ar HaGilgulim p. 95, 114)
Though all of his intentions had been pure, trying only to hold off the Erev Rav (Mixed Multitude) long enough to give Moshe a chance to return to the camp and restore order, still, it had been his hand that had worked the gold. Aharon's was punished for his mistake, to lose all his four sons. Moshe prayed for him and succeeded in saving two of them.
Aharon was later reincarnated as Yaabetz HaShofet (another name for Othniel ben Kenaz), he returned again as Tula ben Puah HaShofet, he reincarnated again in Shmuel HaNavie (Samuel the Prophet), reincarnated again in Aviyah son of King Yerovam, reincarnated again in Uriah HaKohen (who was killed by Yehoyakim the king, and by that was atoned for having to be killed in the Golden Calf incident) and reincarnated again in Zechariah HaNavie. (Shaar HaGilgulim, Hakdama 33)
Aharon's wife Elisheva was later reincarnated as Batsheva, the soul mate of King David. (Kitvei HaAri - Shaar HaPsukim, Shmuel II)
Three good leaders arose for Israel, Moses, Aharon, and Miriam (Ta'anit 9a).There were seventy small courts over which Aharon was appointed (Zohar 3:20:1).
The merit of Moses and Aharon was enough to redeem Israel (Shemot Rabbah 15:3).
Not all are answered when they fall on their faces in prayer, and not all when they rend their garments. Moses and Aharon were answered when they fell on their faces, as it is written, Moses and Aharon fell on their faces (Numbers 14:5), whereas Joshua and Caleb were answered when they rent their garments (Ta'anit 14b).
He loved peace and pursued it, loved people and drew them close to the Torah (Avot 1:12).
Aharon never said to a man or woman, "You have sinned". Moses, however, rebuked them (Sifra Shemini 1).When Aharon went on his way and a wicked person encountered him, Aaron greeted him. The next day, that man wanted to commit a sin, but thought, "Woe is to me! How will I raise my eyes afterwards and look at Aharon? I am ashamed before him, for he greeted me" (Avot d' Rabbi Natan 12:3).
When two people quarreled Aharon went and sat down with one of them and said to him, "My son, know that your friend has said, 'I am ashamed before him because I have sinned against him.' " Aaron would sit with him until he had dispelled the ill feeling from his heart. Then Aaron would go and sit with the other one and say to him, "Know that your friend is saying, 'Woe is to me! How shall I raise my eyes and look at my friend? I am ashamed before him because I have sinned against him.' " Aharon would sit with him until he had dispelled the ill feeling from his heart. When the two friends later met, they embraced and kissed each other (ibid. 12:3).
There were thousands in Israel who were called by the name of Aharon, for if not for Aaron, they would not have come into the world. Aharon made peace between husband and wife so that they came together, and they named the child that was born after him (ibid. 12:3).Aharon tied a rope of iron about his waist and went around to all the houses of Israel. To whoever was ignorant he taught Kriat Shema and prayer, and to whoever could not learn Torah he taught Torah (Tanna d'Bei Eliyahu Rabbah, ed. Meir Ish Shalom).When Moses spoke, Aharon would bend his ear to listen in awe, and Scripture considers it as if he heard directly from the Holy One, Blessed is He (Mechilta Bo 3).
In one year three righteous people died, Moshe, Aharon, and Miriam. Three precious gifts that had been given through them were canceled: the manna, which was in the merit of Moshe, the pillar of cloud, which was in the merit of Aharon, and the well, which was in the merit of Miriam. All of them were removed in the same month (Sifri Devarim 305).
Nehora.com seems to have a great selection of Torah books. I was very impressed. Check them out at http://www.nehora.com/ and add to your library.