I usually enjoy Sophie’s work very much and this was no exception.

To suggest the story is delicate and subtle is to give the wrong impression entirely – and yet there is a deft, floating quality to the prose that is curiously delicate despite the robust storyline.I guess it’s only delicate in the way that watercolours aren’t as forceful as oils, but having said that don’t think ‘pastel’, think ‘cobalt’, ‘viridian’, the vibrant colours of the sea.

Jack Fisher is 16 and orphaned when he is taken by the merman, Vagan, undersea to fight Grimlow, Monster of Darkness, on behalf of the Green Prince.There are traces of Celtic folklore throughout the story – evocations of Pwyll’s meeting with Arawn and his subsequent fight against Hafgan, there’s mermen and selkies and malevolent waterhorses as well as Manannan’s soul cages.

However, the story never stays in one current long enough for any specific myth to dominate.A few surprises along the way – the Green Prince turns out to be less princely than princessly, the malevolent waterhorse has developed an attachment to Lady Linn.The nature of love – as friendship, as romance, ultimately as sacrificial – is explored throughout.

By far the most interesting character was Shellycoat, the sprite of the spring. If I have any quibble at all, it was that the book wasn’t nearly long enough.