I've been reading reviews for Cerulean Sins and have found that the opinions of the readers are split; half find the book just as entertaining as the other novels and the other half does not. I am of the half that find Cerulean Sins extremely entertaining, and believe that Laurell K. Hamilton had done an excellent job. Granted, this installment of the series was not as fast pace as the other novels and sex was a reoccurring theme, just as in Narcissus in Chain. If you don't look at the big picture, then yes this book will most likely be a slight disappointment. Being a writer myself, I understand the big picture. CS is a bridge of sorts. It furthers the plot from Narcissus in Chains, and the character relationships and beliefs that changed in NIC were further developed in CS.
In NIC Anita took the last steps that officially elevated her into the ranks of the "monsters" that she fights on a near daily basis. The lines are blurred for her as to who and what the monsters really are. The other books in the series worked on humanizing the monsters, showing their pasts, vulnerabilities, shortcomings and strongpoint and NIC and CS takes that a step further.
Minor SPOILERS read at your own risk!
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Faithful readers of The Anita Blake Vampire hunter series, this newest installment of the series has to be read, it sets up the plot for the next book as well as the rest of the series. Don't turn your backs on LKH just because this book wasn't what you are used to from her. Continue to read her books with the faith that she wrote CS the way she has for a reason, we just don't know what that reason is yet.
Personally, I enjoyed CS immensely and I can't wait to read Incubus Dreams in September!