Reddit reply in r/AcademicQuran: deleted by user — June 12, 2024 | Ali Shukri Amin

It's a difficult one, not necessarily where I'd recommend starting.

Sayyid Hossein Nasr has a nice concise overview (although I believe he is actually Shi'i leaning)
https://a.co/d/caYBfCG

This is a good practical manual in Sunni Sufism:

https://a.co/d/fFgRUtZ

Sunni Sufism focuses a lot on the diseases of the heart, their cures, taqwa (God consciousness), and Tawbah (repentance). There are many works on these. One of the best is Mohammed Mawlud's, but there is no good English translation (Hamza Yusuf has a subpar one).

Short introduction to Ghazali:

https://a.co/d/hGrRySb

(May be good to also review Niche of Lights, a spiritual commentary on a Quranic passage. It outlines Ghazali's overarching theory of human-Divine relations.)

Ghazali's "Revival" is one of the most important works of Sunnism period and Sufism period. However, it's long, dense, and has complications. There are many critical commentaries in Arabic. In English, go with the revision by ibn al-Jawzi (which is arguably a superior work, but is based on Revival).

Ranks of the Divine Seekers by ibn al-Qayyim
https://a.co/d/2wmGOlb

Hikam
https://a.co/d/1ELinwk

For Rumi, Coleman Barks has an excellent "Essential Rumi". I also really enjoy "Concordance of the Birds" by al-Attar myself, I think it's probably the peak of the Sufi poetic tradition.

As an aside, manners/character development is very important in Sufism. Al-Raghib al-Asfahani's "Enoblements of the Sacred Path" (al-Dhiriyyah fi Makarim al-Shari'ah) is almost the Nichomachean ethics of Islamic thinking. You can find a translation somewhere.