I don't tend to write reviews of products often (in fact, I think this is the first time I've done so), so take that as a mark of how strongly I feel about this book. Left-Handed path contains a lot of information about the Mad, Scelesti, and Tremere (and other soul-eaters) that pushes them from being monster-of-the-week opponents to being complex and scary enough to serve as the subject of an entire story.
Left-Handed Path also goes into a lot of detail concerning how mages view their Left-Handed cousins, and how mages can be seduced by the Left-Handed Path. Useful stuff, for anyone running a game concerned with those sorts of practices and (particularly) Legacies.
The book isn't a catalogue of Legacies, unlike The Sublime and The Ancient, but it does include a few (and expands tremendously on the Tremere Legacy). It's more of a philosophical treatment of the ideology behind the Legacies, treating each of Heretics, the Mad, Scelesti, and Reapers (the new name for soul-eaters of any stripe) as factions unto themselves.
If any of this interests you, I highly recommend this book!