Path of the Incubus is an absolute direct sequel to the previous book Path of the Renegade and if you simply pick this book up without first reading the previous book, well, you might be able to figure some things out, as Andy Chambers does explain who every one is and what they are about, but still, you would be missing out on a very cool read and everything in this book will make a ton more sense.
The title is a tad misleading, as you find yourself following a lot of paths actually, not just that of the incubus, although you will learn that it is his path that is the most crucial to the survival of the dark city. There are 3 main paths, or rather journeys, taking place here. (Although not one of these main 3 journeys I was particularly amused with the two death machines that are programed to hunt down and assassinate someone, that well, I shalln't spoil it!) So, there are 3 paths we follow. First of all, everyone's journey here is a direct consequence to the end of the last book as the dark city of Commorragh ir reeling from the after-effects of a dysjunction that threatens to tear the dark city and it's pocket dimension apart. We see this in the form of daemonic attacks in the lower level that sets two of the previous book's secondary characters, Xagor and Kharbyr, with two rival archons and their parties on a quest to get up to the higher, and deceptively safer (like there is such a thing in Commorragh!) level. This particular journey was one of my favorites to follow and I really like how it ends and sort of sets up the next book.
The 2nd path involves someone we assumed was left for dead in the last book. Needless to say I won't spoil it, but this also sets up the next book too. A favorite aspect of this part of the story is the description and the battle of the Gorath, one of the captured suns or ilmaea that provides heat to Commorragh. Fascinating stuff, and quite an epic battle. Oh, and a personal appearance by Asdrubel Vect that is quite chilling and villainous.
And the 3rd path, the one that really holds this whole story together, yet not directly, is the path of Morr, the incubis from the previous novel and his tag-a-long companion Motley, a harlequin trouper that is determined to convince Morr that he can, and should, personally undo the dysjunction that he caused. This path is quite epic, as we see Morr beat his way through his fellow Incubi and even fight one of the special characters from the Codex! It is also a very well done view on the interaction and attitudes of craftworld elder and their dark kin, as well as the exodites whom we again are treated to more of their culture and background. You could really pull a well rounded ecology article on the exodites from these two books. Also we learn more about the Harlequin thanks to the loose lips of Motley.