Schmidt-Biggemann does not consider any modern perennialists, so there is no mention of Guénon or even Blavatsky. His starting point is the Renaissance, from where he proceeds backwards to Proclus and Plato, and then forwards to Gottfried Wilhelm Leibniz. There is a special focus on the Christian Kabbala, on which Schmidt-Biggemann published a monumental three-volume work, in German, in 2012-2013.
The book is about what Schmidt-Biggemann calls "theological ideas that cannot be separated from philosophical speculation," notably God's self-revelation and the theology of time. This is what gives the book its organisational scheme. Within this, he looks at all the main thinkers of ancient, medieval and early modern perennialism, including--as well as Proclus and Plato--Dionysius the Areopagite, Raymond Lull, Marsilio Ficino, Giovanni Pico, Guillaume Postel, Giordano Bruno, Jakob Böhme, and, finally, Leibniz. He also mentions al-Kindi, whose name he consistently misspells (as "Khindi"), but then he is not a specialist in Arabic thought.
As well as a very wide coverage, the book has some interesting insights. For example:
Seen from the modern perspective of philological historicism, philosophia pernennis was, of course, a syncretistic movement, for it adopted and assimilated all available philosophical topics into its theologico-philosophical system. This was, however, precisely the working idea of perennial philosophy. Since all possible wisdom stemmed from God's original Edenic revelation, no human philosophy could be conceived independent of this origin (xiv-xv).