I honestly didn't know that this was a Young Adult science fiction book when I got it, nor did I discover until later that this the first book of several. That doesn't really impact my thinking here though.
The book stars in a typically typical fashion when our young hero--Jay--accidentally steps into another dimension. Yes there are reasons for that which are eventually covered, but Jay is kept pretty busy for a bit and (of course) accidentally becomes the #1 target for a particularly nasty Bad Guy. Along the way he runs into other versions of himself from other timelines, and it all comes together for a satisfying enough conclusion which (I didn't realize at the time) leaves a couple of hooks for the next book.
Recommended with some caveats: that you like YA fiction and forgive the sometimes McGuffin to tie things together. I liked it myself; I'll round up the others as well to see how the series plays out.