When I read the first Grey Griffins book, The Revenge of the Shadow King, I found it very fun but had mixed feelings about its quality as an actual book. There were many random action events that seemed to come out of nowhere, and many plot elements I felt went underused. On the positive side, I really liked the character interactions and some of the characterizations - at least for Max and Natalia, as Ernie and Harley were underdeveloped.
The sequel deals with all of those complaints, while keeping the strengths from the first book. Right in the first chapter, we learn some of Ernie and Harley's background, and the two are further developed throughout the book, particularly Ernie, who plays some surprisingly important roles.
The action picks up very quickly. I won't spoil what happens, but I will say that this time, there's far fewer random elements and much more things that connect together, with a more cohesive overall plot, and many surprises. I was genuinely surprised and impressed.
Natalia retains her inquisitive demeanor and love of snooping. Max is, like before, more interested in being with friends and family than he is with his family's massive wealth. His personal problems are explored in greater detail than anyone else's backstory, as he is the primary main character after all.
The story does follow roughly the same formula as the previous. The four friends go on adventures, get rescued by adults, have some backstory dumped on them (there's one entire chapter that's mostly dialog and backstory, with no action), and the final adventure involves the kids going at it alone. Only this time, the formula has been tweaked to near perfection. There's more logical reasons for why these things happen, and it just feels more designed and well thought out.
If this is a sign of things to come, this series could well pick up. The series' trademarks - lots of friendly banter between the kids, plot dumping in the form of dialog from adult mentors, and lots of close calls - will probably remain consistent throughout. But as long as these trademarks and handled with care so that they form a well flowing narrative rather than a mixed bag, the result will be a lot of fun.