Harry Potter and the Order of the Phoenix, by JK Rowling

For much of its punishing length, Harry Potter and the Order of the Phoenix shows how the fifth-form wizard discovers, and deals with, his dark side: his anger, his arrogance, his aggression. Heroic Harry becomes Dirty Harry, even subject to a lifelong ban from Quidditch after he thuggishly wraps his fist around the Snitch and tries to punch "every inch of Malfoy he could reach".

This is a consistently bad-tempered book, in which our troubled hero runs the gamut of grouchiness between "growling resentment" and "white-hot anger" as he persuades the wizard world that Voldemort has come back, and loses a dear friend. Combined with its bulk, this atmosphere makes for a fairly downbeat read. No one can accuse Rowling of shirking the acrimony of adolescence, from the nasty run-in with cousin Dudley to the "great rush of hatred" when Harry spots Snape.

The Order of the Phoenix has a strange, uneven shape. We don't get to Hogwarts until page 182, after a detour in the ancestral town house of Harry's godfather, Sirius Black: still branded a criminal by the Ministry of Magic. Here, the titular order (dedicated to the defeat of Voldemort) hides its dilapidated HQ. So Harry gains a new gang of wizards who include a cool, ear-ringed black dude and a purple-haired punk. In an oddly premature, half-cock court scene, he also has to defend himself against a made-up misuse-of-magic charge.

Once the Hogwarts term begins, we lose the intriguingly adult Order members until a Hollywood-style showdown with Voldemort and his Death Eaters. This climax feels more Spielberg than Rowling, but we do return to Dumbledore's study for a long session of recap-and-prediction. On the Voldemort front, it is remarkable how little these 766 pages extend the overall story-arc, of his return and Harry's destiny. We do learn, from a prophecy, that Harry must become either slayer or victim of the Dark Lord. What will the true source of their eerie intimacy turn out to be? Folk-tale logic suggests something like a grandfather-grandson bond.

In between preface and coda is an almost self-contained, 500-page Hogwarts novel. Harry does see some "hormonal" action with Cho Chang, although when he gets to snog her under the mistletoe, the narrative trails off into an evasive row of dots, a Mills-and-Boonish solution to the ever-deepening problem of how to reconcile her younger and teenage readers.

Some of the Hogwarts section drags. Rowling is ringing elaborate changes on familiar motifs: Hagrid and his giant relations, house rivalries, magical creatures, preparations for the OWL (Ordinary Wizard Level) exams. Her pretty ordinary prose reaches the tedious side of functional.

Harry's stomach is invariably lurching or sinking as a token of his (usually outraged) feelings. Meanwhile, that Quidditch ban, and the truncated match descriptions, prove how much the game now bores her.

The most appealing Hogwarts strand dramatises the rise and fall of the atrocious Dolores Umbridge. This ministry snitch spies on every teacher, and briefly usurps Dumbledore's role as head. Toad-faced, pink-cardiganed, Umbridge is a triumph: a New Labour-style interfering bureaucrat who stamps on independent teaching and witters on about "a new era of openness, effectiveness and accountability". If her decrees don't appear photocopied on every school noticeboard in Britain, then I'm a Hippogriff.

Umbridge also loathes "filthy half-breeds". A fascistic devotion to "pure blood" marks the the sadist, the crook ... or the pitiable loser. At the Black mansion we meet the house-elf Kreacher, a deluded retainer who upholds the racial bigotry of the elite. Dumbledore reignites Harry's rage by pointing out that his mentor Sirius exploited Kreacher and his class. For, in the finale, Kreacher the servile worm turns, and delivers beloved Sirius into the power of You Know Who ...

Dumbledore labours to make Harry see how the world might look through Kreacher's eyes. Throughout the book, Rowling dwells - rather impressively - on what we can know about the inside of other people's heads. On the magical plane, Harry and Voldemort share an ever-more telepathic relationship. The scar on his forehead now gives regular access to the Dark Lord's emotions of exultation, cruelty and despair. Harry fears he is turning into "a kind of aerial", always tuned to Voldemort FM. Dangerously, this psychic traffic can move both ways.

Elsewhere, the theme of "other minds" leads to the mystery of girls. Hermione does her best to explain, but "feelings and stuff" still trap Harry. A genuine breakthrough comes when Harry looks into Snape's "pensieve" (a safe for secret thoughts) and sees the teacher's school-age memory of humiliation at the hands of Harry's late father and Sirius. Seen through Snape's eyes, this pair (whom Harry idolises) show that being "the height of cool" can also mean behaving like "arrogant little berks".

This chapter encapsulates the drift of the Potter series away from moral blacks-and-whites, into ever-shifting shades of grey. We even grasp that the ghastly Dursley household has a role to play in keeping Harry safe. From the start, when rogue Dementors descend on Privet Drive, the book shows Harry reacting to breaches in "the great, invisible wall" he once thought separated good and bad, love and hate, self and others, magic and Muggle. At present, he responds to this "contamination" with confused annoyance and anger. Part six should herald a change of mood. By page 766, exhausted readers will be hoping for a change of pace as well.