I have not read this book in its entirety, yet I rated it five for its excellent section on Rumi's theology.
If you have been reading a lot of philosophy books and find yourself frustrated at the transiency of truth (small t) approached from strictly rational perspective (Foucault give up on philosophy and history and invented genealogy, didn't he?), reading Rumi's Theology chapter in this book may grant you relief: Truth can only be found through Divine Revelation.
Schimmel's narration on Rumi's theology gives you a flavor of traditional Islam's perspective on theology. For one, Rumi rejects anthropomorphism (i.e. equating God with His creation, such as that God has hand and feet like human and sits in His Throne like human being) argued by certain "scholars". This is "as blameworthy as calling a man Fatema. What is an honour for woman is a shame for man; and although hands and feet are an honour for human beings, it does not aply to the Divine Being" (p. 237). Another is that remembrance of God (dhikr), individually or in a group, is at the heart of traditional Islam's practice.
A key section in this chapter offers a glimpse on Sufism view of man-God relation through the exploration of "Alast." "Alast" is a shorthand for the famous verse in the Qur'an (Sura 7/171) in which The Divine addressed the future humanity when we were still "concealed in the loins of the yet uncreated Adam." God said "Alasto bi-rabbikum?" Am I not your Lord? To which all unborn souls answered "balaa, shahedna." Yes, we witness it. Schimmel repeats Rumi endearingly tha, "The Divine alast and the human answer is the seal of the heart" (which is, of course, come from Rumi's writing). This is the Primordial Covenant.
Our ache, longing, and loneliness roots back to this separation from God. It is only by going back to God, by reawakening the soul to this Primordial Covenant, that we find rest.
This section of Triumphal Sun is truly a gem worth reading and re-reading for those who are yet to appease that ache, as much as for those who have started walking the path to God.