I am looking for recommendations for a good online mussar work to study with a beginner (english).
How about Mesilat Yesharim, one of the most commonly studied mussar text?
Available online at Sefaria. Also available as an ebook from artscroll for a few dollars, in the style of the Schottenstein Gemara. Using the latter made a huge difference for me in terms of making the text come alive, relevant to the 21st century and that much easier to understand. Highly recommended for a beginner.
See also this excellent answer from miyodeya on why Mesilat Yesharim is so popular especially the point that
[the author] continually stresses that every positive step, no matter how small, is actually a major achievement, and that even one who attains to only the lowest of the levels described in the book has done something extraordinary.
As you had suggested in your comment, Pirkei Avot has numerous adages regarding ethical behavior, Torah study, etc. It is good, but for a beginner, not really focused on one specific area.
I think that a better, more focused approach would be the beginning two sub-volumes in Rambam's (Maimonedes) Mishnah Torah - Sefer Mada, spec.
Hilchot Yesodei Hatorah - these deal with essential Judaic beliefs of G-d, faith and Torah andLaws of Torah Foundations and
Hilchot De'ot - this volume deals with human behavior such as avoiding anger, how to relate to others, etc.
English versions for both books are available on the Chabad web siteor on Sefaria.org. Sefaria, though, doesn't have English for everything, and, I have found some inaccuracies in the translations. Chabad's site, at least for Ramba"m seems more complete, and I think, more accurate.
I just found this and I like it (it is in alef Bais order)
Pele Yoetz is a book of Jewish Musar literature first published in Constantinople in 1824 by Rabbi Eliezer Papo
If you want somthing very logical then i love
Hebrew http://www.daat.ac.il/daat/mahshevt/hovot/tohen-2.htm Chovot HaLevavot, or Ḥobot HaLebabot, is the primary work of the Jewish philosopher Bahya ibn Paquda, fullname Bahya ben Joseph ibn Pakuda
Maybe Kuzari https://en.m.wikipedia.org/wiki/Kuzari will be good
I would suggest cheshbon hanefesh, with English commentary by Feldheim publishers. It's very practical and very deep; it can be learned on many levels. The author, Rav Mendel Lefin, himself had a very interesting background. Rav Yisrael Salanter suggested that the sefer be reprinted and wanted it to become more widespread. I am such an avid fan of the book that after learning it for several years now, I began a blog based on the book, and you can buy the book online.
Lev Eliyahu translated into English by Klein, not sure if this is available online though.
Or Strive for Truth (Michtav me'Eliyahu in English), probably also not available online.
Or the first volume of Tenuas ha'Mussar translated into English (in English this is 3 volumes, definitelty not online though, but then Reb Yisrael Salanter also didn't have an iPhone).