Rabbinic Mussar Literature
Mussar is a genre of Jewish literature (ספרי מוסר) that started in the Bible and began to take rough shape in the next thousand years from approximately 500 BCE to 500 CE, the Rabbinic Era. The following are some of the Rabbinic writings containing at least some Mussar content from that time, along with later commentaries on them. This whole collection of 100+ Mussar books is also available as a PDF here.
Pirkei Avot (פרקי אבות)
Ethics of the Fathers or Pirkei Avot (פרקי אבות) is a collection of teachings from the ancient Jewish sages. It's often simply called "Avot" or "Fathers/Parents" because of its parental Mussar instruction. One of the 63 tractates of the Mishnah, it was written down in the late 100s. Often a sixth chapter from the Talmud with similar content is appended.
Get Artscroll's lightweight 1999 version of Pirkei Avos: Ethics of the Fathers with concise commentary, edited by Rabbis Meir Zlotowitz and Nosson Scherman. 64 pages.
For more commentary see Artscroll's The Pirkei Avos Treasury: The Sages' Guide to Living With an Anthologized Commentary and Anecdotes published in 1995 and edited by Rabbi Moshe Lieber. There's also the smaller 1996 three-volume slip-cased set. 439 pages.
If you like pictures get Artscroll's 2015 Children's Pirkei Avos, written by Shmuel Blitz and illustrated by Chani Judowitz. 108 pages.
For pictures there's also Artscroll's 2005 Youth Pirkei Avos in hardcover and paperback, written by Rabbi Avie Gold and illustrated by Michael Horen and Andras Halasz. This single 88-page volume appears to be a reprint of the original 1989 two-volume set with Volume 1 covering Chapters 1-3 and Volume 2 covering Chapters 4-6. 48 pages each.
If you prefer Koren get the beautiful 2015 Koren Pirkei Avot translation by Rabbi Jonathan Sacks with commentary from Rabbi Marc Angel.
Koren also offers Sage Advice: Pirkei Avot, a 2016 version with translation and commentary "framed by thumbnail sketches of the lives and times of the sages" from Rabbi Irving Yitz Greenberg. 378 pages.
Avot d'Rabbi Natan (אבות דרבי נתן)
Ethics of Rabbi Nathan or Avot d'Rabbi Natan (אבות דרבי נתן) is basically a sequel to the Ethics of the Fathers from roughly the same time and is tucked away as a minor tractate in the Talmud. It's hard to find all the versions because no one spells it the same. Case in point, the above is Artscroll's 2017 Avos DeRabbi Nassan: The ancient baraisa that illuminates the teachings of Pirkei Avos. Along with extensive commentary this version also includes the complete text of Pirkei Avot. 660 pages.
If you'd like a shorter version there's Avos D'Rebbi Nosson, translated by Rabbi Avraham Yaakov Finkel. 229 pages. Originally published in 2006 by Yeshivath Beth Moshe as Volume 1 and Volume 2, they were combined into a single volume by Judaica Press in 2007.
Tana D'vei Eliyahu (תנא דבי אליהו)
Elijah's Teachings or Tana Devei Eliyahu (תנא דבי אליהו) is a Mussar-toned Midrashic work from the early Rabbinic era. Get the English-only Tanna Debe Eliyyahu: The Lore of the School of Elijah translated by William Braude and Israel Kapstein and published by the Jewish Publication Society in 1997. 642 pages. There's also the original 1981 JPS edition, available here and here.
Barely worth mentioning is Tanna Devei Eliyahu: Teachings from Eliyahu HaNavi translated and annotated by Rabbi Avraham Yaakov Finkel and published in 2013 by Yeshivath Beth Moshe. This is assumedly Volume 1 but there's no ISBN. Volume 2 can be confirmed through ISBN search. And Volume 3 is also confirmable through ISBN search. This listing claims to also be Volume 3. The last 9 chapters of this work were never completed. In all cases, puchase at your own risk.
Midrash Tanhuma (מדרש תנחומא)
Tanchuma's Expositions or Midrash Tanchuma (מדרש תנחומא) is a collection of Midrashic discussions from the early Rabbinic era opening with quotes from Rabbi Tanchuma and containing at least some Mussar teaching. Midrash Tanhuma: Translated Into English with Introduction, Indices, and Brief Notes translated by John T. Townsend was published by Ktav Publishing as Volume 1 (Genesis, 334 pages, 1989), Volume 2 (Exodus and Leviticus, 394 pages, 1997), and Volume 3 (Numbers and Deuteronomy, 394 pages, 2003).
Samuel Berman also did a partial translation as Midrash Tanhuma-Yelammedenu: An English Translation of Genesis and Exodus with an Introduction, Notes, and Indexes. 702 pages, published by Ktav Publishing in 1996.
Pesikta d'Rav Kahana (פסיקתא דרב כהנא )
Rav Kahana's Verses or Pesikta d'Rav Kahana (פסיקתא דרב כהנא) is a collection of discourses with opening statements from Rabbi Kahana for Shabbats and Festivals and is one of the earliest Midrashic works with Mussar material. Get Pesikta De-Rab Kahana, translated by Rabbi William Gordon Braude and Israel James Kapstein and published by the Jewish Publication Society in 2002. 864 pages. This is a reprint of the original 1975 593-page edition published by the Jewish Publication Society here and another publisher here.
There's also Jacob Neusner's Pesiqta De-Rab Kahana: An Analytical Translation, published by Scholars Press in 1987 as issues #122 and #123 of the Brown Judaic Studies series. Volume 1 is Chapters 1-14 and Volume 2 is chapters 15-28 with Introduction. 245 and 274 pages respectively. You can also get this as a two-volume set.
Pesikta Rabbati (פסיקתא רבתי)
The Big Pesikta or Pesikta Rabbati (פסיקתא רבתי) incorporates passages from Pesikta d'Rav Kahana and Tanchuma and also includes new material which features some Mussar content. Get Pesikta Rabbati: Homiletical Discourses for Festal Days and Special Sabbaths, translated by William G. Braude and published in 1968 by Yale University Press. 1050 pages.
Derech Eretz Zuta (דרך ארץ זוטא)
"Little Book of Etiquette" is a loose translation of Derech Eretz Zutta (דרך ארץ זוטא), a minor tractate tucked into a corner of the Talmud with 13 other noncanonical works including Pirkei d'Rabbi Natan. It's especially famous for its last 'Chapter of Peace' (פרק השלום/Perek HaShalom). For all 14 Minor Tractates there are Soncino's One-Volume Hebrew-English editions from 1965 and 1984 and their Two-Volume editions from 1965 and 1971. Also search the ISBN. 1442 pages. You can also read it on Sefaria.
Shmoneh Prakim (שמונה פרקים)
There are Eight Chapters or Shemoneh Perakim (שמונה פרקים) in the Rambam's code of Jewish Law known as Mishneh Torah commenting on Pirkei Avot, the first book in this list. Get Moznaim's 1994 edition. 184 pages.
Rabbeinu Yonah (רבנו יונה גירונדי)
Rabbeinu Yonah Gerondi's commentary on Pirkei Avot was written in the mid-1200s. Get Rabbi David Sedley's 2008 translation published by Judaica Press. 352 pages.
Ovadiah Bartenura (עובדיה מברטנורא)
Rabbi Ovadiah Bartenura's commentary on the Mishnah has been standard fare since the late 1400s. Read the only English translation of his commentary on Pirkei Avot, courtesy of Rabbi Robert Alpert, on Sefaria.
Midrash Shmuel (מדרש שמואל)
More available is Mosaica Press's 2020 translation with commentary by Rabbi Avrohom Yaakov Rodkin. 237 pages.
Chafetz Chaim (חפץ חיים)
Rabbi Yisrael Meir Kagan (רבי ישראל מאיר כהן), popularly known as the Chafetz Chaim (חפץ חיים), wrote many Mussar-toned books and was an ally of Rabbi Israel Salanter. He wrote this commentary on Pirkei Avot, along with Avot d'Rabbi Natan and Tanna Devei Eliyahu, in the late 1800s. Get Feldheim's 2002 translation. 364 pages.