"Watson could feel the cloud of gloom creeping over him, as insidious as any poison gas."

There’s darkness in this novel, the fourth in Robert Ryan’s excellent Dr Watson series.

Darkness from the skies as night bombing rips into the heart of London, darkness in the madness of the Front as one of Watson’s friends disappears without trace. And there’s a particularly malevolent darkness on the streets of the city as eminent gents like our good Doctor’s concert-going friend Sir Gilbert Hardy are kidnapped – bundled away to a very uncertain fate.

And there’s a darkness in John Watson himself. The events of the previous novel in the series weigh heavily on him – you don’t need to read these books in sequence, but I’d recommend it – and also it seems in the picture of his whole life, his relationships, his time with Holmes, his sense of his own era’s place in history. There’s a real feeling of reckoning in the room as Dr John Watson tots up his tab – a study of grief in all things, of time passing.

The Sign of Fear – nice touch that title – doesn’t flinch from these deep things, but it’s also really good entertainment, and well deserves its publisher sticker ‘A Dr Watson Thriller’. For a man in his sixties, Watson fairly gets thrown about the place. He also has to deal with some pretty desperate characters in this book and faces down oblivion in his customary style. Ryan excels in capturing the robust bravery of Watson – his doggedness and determination to get things done – which is such a strong part of the source character’s appeal in the original Conan Doyle. It’s hard to get that quite right, and for me Ryan does it well.

I particularly liked the parallel narrative involving the German pilot Schrader – it’s woven through the story and Ryan’s expert but even-handed use of technical detail really gets the balance of information right. The story of Watson’s missing friend is quite the most stunning part of the book – indeed it’s the storyline that will most sustain for me – but, no spoilers on that one. Ryan achieves something very difficult with this layering of elements, so that the conclusions the book reaches – just as you might think its ends are fraying – are very effective indeed.

So, The Sign of Fear is impeccably researched (World War One is vividly revisited – particularly the air raids over London), it has an emotional depth as well as lots of gunshots and derring-do – and as he has across this acclaimed series, Ryan creates a very impressive and sympathetic Watson. It’s good stuff.

The Sign of Fear is published by Simon & Schuster, £7.99 in paperback