Corbin Cyclical Time and Ismaili Gnosis by Henry Corbin
The book brings together three essays of Corbin's translated from French.
1) Cyclical Time in Mazdaism and Ismailism - from the Eranos conference in 1951
2) Divine Epiphany and Spiritual Birth in Ismailian Gnosis - from the Eranos conference in 1954
3) From the Gnosis of Antiquity to Ismaili Gnosis - from a conference in Rome in 1956
Mr. Corbin discusses the Ismaili ideas of cyclical time, yes. The Ismailis are often associated with the Alamut Assassins of lore but they are so much more than that. He also digs into the ideas of the Qiyamat (the Resurrection) and the beliefs held by the Shi'ites of the Imam-of-one's-own-being and a whole host of other ideas that fall loosely under the banner of the Ismailis.
The essence of the book is a dissection of 'gnosticism' and its trajectory from Zoroastrianism/Mazdaism through Greek philosophy in its intercourse with Avicenna in the Islamic world through its eventual and perhaps ultimate manifestation in Shi'ism. It's something of an alternative view to an Incarnationist Christology, an attempt to recover something 'lost' by the worldview of this Christianity of the Councils.
The bonus is that you will learn more about all those variant beliefs to be found within Christianity, Jewish and Islamic history and the deeper elements of Zoroastrians, Gnostics, the Ebionites, the Manichaens, the Sabeans and other divergent groups and groups within groups that you may never have even heard of.
And these are all interwoven into one grand synthesis that is mind blowing. It's not an easy read. I've read through this twice and am now trying to digest it a third time. It's still dizzying.
Yet the work is worth it. There are moments of lucidity that make the journey valuable. This is not for mainstream Christians, Muslims or any others who cling solely to the exoteric traditions. If you are happy there, stay there. This is for those who find something amiss in simple religion and dare to go beyond (or within). But be warned, it's not for the fainthearted.
To my knowledge, this is the only work of its kind in English, at least of this depth and magnitude. If you really wish for a deeper view of the spiritual universe, all of Corbin's books come recommended.