To that end, this introduction to Voegelin is a great place to start cracking into a dense philosophical writer. In fact, I would suggest reading this before attempting any actual Voegelin, otherwise you'll run the risk of getting lost in Voegelin's made-up terminology and self-referential thought. Once you've got this book under your belt, then pick up Israel and Revelation or some such and dive in...
Voegelin aside (and despite what it sounds like here, if you're interested in the 20th century Voegelin is worth at least some attention), Federici's book is well-written, interesting, and thorough in its treatment of the subject matter. Beginning with a brief biography, Federici walks the reader through Voegelin's overall project, his history of philosophy, his philosophy of history, his political theory, his philosophy of consciousness (the place where most people will need help, I suspect), the critics of Voegelin, and Voegelin's contribution to the discipline of politics. Perhaps most useful in the long-term is a glossary of Voegelinian terms that I will certainly be referencing when I go back to some of texts I probably should have read in grad school.
Just to give you hint of how necessary something like this introduction (and the glossary) is, take the example of Gnosticism. This is one of Voegelin's nemeses, but it has nothing to do with Nag Hammadi. Instead, by "Gnosticism," Voegelin means:
An ideology that claims absolute knowledge of reality. It characterizes the modern world according to Voegelin. It is engendered by dissatisfaction with the structure of existence as it is and the belief that a new order can be created by implementing a revolutionary plan of action based on gnosis. The new order represents a transformation of human nature and the very structure of existence. (219)So clearly we're dealing with an in-language that needs some clarification if one is to really get at what Voegelin is doing.
With that said, it is worthwhile to get at what Voegelin is doing, and Federici has provided us with an excellent means of doing so.
Recommended for those interested in trends in 20th century philosophy.