Penny Dreadful is an irreverent and untamed adventure that uses the gloomy and serious game-world to great comedic effect.
The book crosses over with several of the World of Darkness lines, but while knowledge of them might clear a few things, I don't think it is necessary. Even knowledge of Mage the Ascension isn't all that critical, since Penny makes a point of explaining her magic to herself and others.
As the humor goes, Dreadful is very reliant on references. But these are delivered through the first person account of Penny herself, who uses 90's pop-culture, Victorian goth and medieval occult together to cast her magic as well as make sense of the world around her. And it works. These references are all over the place and Sailor Moon is just as prominent as the Malleus Maleficarum and familiarity with either is not essential, but contributes to the hilarity. Symbolism is the name of the game and Penny's habit of substituting Bastet, the cat goddess of ancient Egypt for Hello Kitty and similar such juxtapositions, fuels much of the book's charm.
The book isn't dark, but it is crass. From Penny's veiled threat to curse a vampire to switch from blood to excrement to the theft of a sexual organ as one of the major plot points. Often Penny does a "sweet Victorian lady" routine and leaves the reader (and other characters) to guess her all to clear sexual allusions. But other times, and especially in the last chapter, all reigns are loose and kinky sex just oozes from the page. That last chapter as a whole is an unexpected shift in tone, but nothing that stands really at odds with the rest of the book.
While I blushed more then once at some of the more lurid descriptions, I enjoyed Penny Dreadful very much. It is funny and makes good use of the source material. The 90's saturated atmosphere defiantly played on my nostalgia and I liked the odd mix of pop-culture and historical academic trivia.
From a game perspective: Penny shows how magical paradigm can work for individuals on the edge of the Traditions' society. Penny's crafts are cobbled together from pop-spirituality, new-age mysticism and bits of more established magical traditions. The book does a good job of showing how such a paradigm works not only for magic but also for the way Penny assumes the world around her works. It's a good example of Hollow Ones' practices and how a street level Mage game might play.
It's short, fun and heartily recommended, if you don't mind a little hard kink in your comedy.