Update 11/3/13 - downgrading this to 3.5 stars. I did like it but I don't think it deserved to be rated the same as Hollow Earth, to which I awarded 4*
I'm going to be generous and give this 4 stars because I truly did enjoy the last few chapters. However, for me much of this second book in the Hollow Earth series was slow going, without the consistent page-turning quality and excitement of the first book. I think it took until I was 48% through it until I really started to enjoy the story. That said, it's still an excellent series and after such a thrilling cliffhanger, I will definitely be picking up book 3 when it comes out.