I initially found the fact that this "mystery" is 369 pages rather daunting, but it's highly readable, and quickly drew me into it: a hundred pages had passed before I realized it! It gives you a vivid picture of what Paris was like during la décadence, along with an entertaining semi-fictionalized treatment of Wilde's 1882 American tour. As is typical in books like this, the murder mystery plot is really just an excuse (though I found it clever, if a bit far-fetched). The book is really about what it would be like to have a wit like Oscar Wilde as a friend, and what life was like in the circles he moved in. There are two earlier books in the same series (it's coming out at six-month intervals) which I missed, and which I plan to read as soon as I can.
Brandreth is a Wilde student, and a student of the period, and he certainly has it down cold. He sometimes seems to be challenging his reader to say "hey, that's an anachronism." He makes half a dozen references to Lucky Strike cigarettes, for example, but a little research shows that they were indeed popular in the period. If Brandreth made any slipups, I couldn't catch them.
Surprisingly, Wilde's homosexuality isn't mentioned at all, except for a couple of vague hints. The reason is that the story is told through the eyes of Wilde's friend Robert Sherard, a real person, best known for his later Wilde biography which soft-pedals this aspect of Wilde's life (astonishingly, Sherard claimed to be ignorant of it until the scandal broke in 1895). This is the third of nine (!) books promised in this series, and I look forward to seeing what happens in the later books when Oscar starts feasting with panthers.