The short answer is biased toward the former, with a caveat or two.

Mostly, the continuing adventures of PC Grant bumps and grinds nicely enough. But there's the rub. The tag line used at the beginning of this series was 'what if Harry Potter grew up and joined the Fuzz?'

Harry Potter, by book three, was absolutely on fire. Indeed, there are many readers who concur that Harry Potter and the Prisoner of Azkaban was the best of J.K. Rowling's novels. In contrast, Whispers Under Ground merely advances Peter Grant's story at a workman-like pace, and sets a few scenes for what are presumably bigger, deeper things to come. The greatest depths here are the Morlock-like tunnels which Grant & Co. find themselves exploring during the course of their investigations into a suspicious death. On the plus side, the author's style of writing flows nicely and retains is unique flavour. The natural, realistic humour employed often delights, and the ever expanding cast helps to reveal more about London's magical past. On the negative side, I felt that this tale was less engrossing than the previous two, and that was mostly due to plot lines that were deliberately put on hold. Specifically, the rogue magician who Grant encountered in book two, and all that his origin suggested. Yes, there is reference to this, and a crafty sub-plot which keeps the pot boiling. But, this is very much secondary to the main thread, and felt to me like a holding tactic. This is not to say that the new characters and directions are boring or without merit. Indeed, they do help to give Grant's semi-magical world an extra dimension. It was just not what I'd anticipated. There's also the issue of how slowly Aaronovitch's central character is developing in a magical sense, and the continual sparse use of Nightingale, the Met's only master magician. Is the development realistic, yes it is. But this is fiction, not a documentary, and a little more artistic licence would not go amiss. Don't get me wrong, I'm not suggesting that PC Grant starts riding a Nimbus broomstick, only that delving deeper into the world of magic -- in particular Nightingale's past -- would be most welcome. Part of me, I must admit, would prefer to be reading about Nightingale as the main character, with Grant, and the increasingly excellent Lesley May, as his plucky young apprentices. But that is not the series that Ben Aaronovitch has chosen to write.

Whispers Under Ground is a good book, not a great one. The non-magical cast continue to evolve in fun ways, the situations remain mostly realistic, and the metaphysically inclined trio of Grant, Lesley May and Nightingale make progress, albeit a little too slowly for my liking. In summary, those who have already read the first two titles in this series will find enough here to keep them interested. But new readers should start at the beginning.