I'm a huge fan of the series, and this book is much the same as what has come before.
It is still an absolute pleasure to see the characterisation of Oscar Wilde, as charming as ever, and this book has the happy advantage of making more of his relationship with Arthur Conan Doyle, as the writer still attempts to flee from the success of his Sherlock Holmes stories.
The vatican provides a nice change of scenery, and as ever the author does a fantastic job of really painting the victorian lifestyle so that you feel part of it and it feels both simultaneously fanciful and yet believable.
All of these are fantastic features, but have been done -in some form or another- before in his previous books to more quality. By this book (the fifth in the series)everything seems to be getting a little tired, and the author could benefit from some new ideas. Most of the twists have been done before in the previous books, and it seems to be nudging a little bit more towards a 'paint-by-numbers' mystery.
Generally speaking, if you are already a fan of the series, you are sure to enjoy it though perhaps will not be as charmed or as engaged as the last books. Everything you like is there, but it seems to lack some of the spirit and certainly the originality of the previous stories.
If you are new to the series, give this book a miss and backtrack to 'Oscar Wilde and the Candlelight Murders' (the first book) and start there. The first four books are all wonderful and you're sure to enjoy them.