SEEKER. By William Nicholson. Egmont pound;12.99

THE BARTIMAEUS TRILOGY III: Ptolemy’s Gate. By Jonathan Stroud. Doubleday pound;12.99

Nicholas Tucker tackles fantasy trilogies from both ends of the spectrum

Modern-day mermaids are not easy to accept when underwater photography reveals shapes and sizes more dramatic than anything a human imagination can dream up. But Helen Dunmore still makes a good case for the existence of the aquatic, alternative world of Mer somewhere off the shores of Cornwall, only occasionally getting bogged down in less convincing practical detail.

The first of a trilogy, Ingo concentrates on young Sapphy and her older brother Conor, whose fisherman father disappears one night, never to return. Slowly the two children realise that like their father they too have something of the magical world of Mer about them. Each gets lured deep under the sea, there to experience an excitement and freedom that makes their home seem increasingly remote and uninteresting. But once they return to earth and re-experience their loyalties there, each then tries to save the other from following their father into watery oblivion.

Matters come to a head when Mum’s new boyfriend goes diving on the Mer’s most sacred spot. Limpidly written with a poet’s eye for detail, this unusual story combines the realities of sibling rivalry and adolescent jealousy with the fantasy of what initially looks like an ideal world elsewhere, before its own darker side slowly reveals itself.

William Nicholson’s Seeker is also the first of a sequence of three novels, set in an imaginary world where an island populated by noble warriors known as the Nomana is threatened by a neighbouring country ruled by a brutal oligarchy. There are touches of Orwell’s 1984 when the King of this vile set-up goes for his daily hate training sessions, but in general this is an original, inventive story, cleverly plotted and full of interesting characters.

Its Quakerish message of peace and love is accompanied by a tough-minded realism when it comes to disposing of villains clearly beyond redemption, making any final judgement about what truly constitutes good and evil in the context of this particular story necessarily hard to decide.

Parallels with the world of today, from would-be suicide bombers to ghoulish public executions, demonstrate how fantasy can be quite as confrontational as any other sort of writing. Its two main characters, a boy and girl both aged 16, remain psychologically recognisable despite their supernatural gifts. Although everything ends perhaps a little too happily at the end of this instalment, there is clearly lots of trouble still to come for readers patient enough to wait for the next two volumes.

He’s back! Bartimaeus, the world’s most irritable djinni and undisputed master of the withering footnote, makes his final appearance in Ptolemy’s Gate, the third volume in Jonathan Stroud’s intelligent post-Tolkien saga, The Bartimaeus Trilogy.

With irony replacing rhetoric, these excellent books manage to be consistently gripping and amusing at the same time. Three years have passed since the previous confrontation with Prime Minister Gladstone, also a powerful wizard to those in the know. Now Nathaniel, the jumped-up young magician only just in charge of his brief, finds that the greedy oligarchy dealing exclusively with magic that he has always yearned to belong to is on the brink of destruction. One of its enemies is young Kitty, a resistance fighter drawn from the ranks of the virtually enslaved commoners.

Enter Bartimaeus once again, on no one’s side but his own but still linked to Nathaniel by ties he would so much rather escape. Invaded by armies of demons bringing about fearful carnage, the world seems at risk until Nathaniel and Bartimaeus, urged on by Kitty, temporarily become as one, each prompting their other half as they set about driving back the enemy with the aid of their most lethal weapon, Gladstone’s Staff. But their victory has its costs, and in the final pages the satirical spirit running through these books is replaced by something much sadder. Like all good authors, Stroud demonstrates here that what and how he writes should never be taken for granted, least of all when he is bringing to a close a trilogy that is his finest achievement yet.