The author already mentioned that the story would only really start with book 4, that all the books so far were more of an introduction. Still, "Zom-B City" was sort of a let-down for me as it felt too much like a filler book. There isn't much going on; we get introduced to the most important structures in London that were established after the zombie attack and that's that. Basically, it's B roaming the city.
Therefore, the book wasn't exactly boring, but not very thrilling either. Only a few scenes in between and the cliffhanger in the end got me again and that's why I'm looking forward to "Zom-B Angels". Hopefully, it will have more of a plot to offer; so far the series was very good after all.