In "The Lies of Locke Lamora", to which this is the sequel, Scott Lynch introduced a band of con artists and sneak thieves in a lush fantasy setting. That book showed some real promise, despite a slightly clunky structure and a heist-with-a-twist-at-the-end plot which didn't quite work. Unfortunately, Lynch is apparently determined to parlay that into a seven book series, and this is probably why "Red Seas under Red Skies" fails.
The surviving gang members from the first book, the silver tongued Locke Lamora and tough Jean Tannen, have fled to pastures new and are plotting a new heist while being hunted by dangerous enemies. So obviously they have chosen a long, complicated and dangerous rip-off of a gambling house that is famous for never having been robbed. Unfortunately, having set up this potentially interesting plot, the book then tacks hard in another direction, throwing them into a convoluted political game involving them pretending to be pirates. All the best heist stories are complex, with layers within layers, but the secret of success is that all the complications flow from one another, driven by character conflict. Instead, Lynch just throws external threats in every time he needs a new twist, and so the whole impression is of confusion.
However, Lynch can write amusing characters and dialogue, and he makes sure there's a good sense of pace. Lamora spends a bit too much time wallowing in grief due to past events to be as amusing as he can be, and we don't get as good a feel for the locations here as we did the city of Camorr in Book One, but his world is enticingly different.
The main flaw is that rather than writing a self-contained tale, Lynch is slowly juggling elements for further books, and it's this obsession with taking up so much of our bookshelves that stops him and many other promising authors from producing their best work. This book is worth a read, but Lynch needs to start writing a story for each book, rather than stretching for a series.