Two years ago, Joey Harker stumbled into another dimension and ended up joining Interworld - an elite fighting force consisting of versions of himself from multiple Earths who can walk between worlds and strive to prevent the bad guys from taking over the Earths of all the various dimensions.
In The Silver Dream, new characters are introduced. Most significant is Acacia Jones. She has the ability to "walk" between worlds but is clearly not another version of Joey.
During a training mission, Joey loses a teammate, discovers there is a traitor within Interworld, and is transported through time.
This book is the sequel to Interworld and suffers from even less Gaiman than the first novel. The writing itself was mostly delegated to Mallory Reaves, daughter of Interworld co-author Michael Reaves.
This is a good Young Adult book and a quick read, but well below what Neil Gaiman fans have come to expect. And it ends with a cliffhanger, which pretty much means you must read book 3 to finish the story.