I read an old novel recently, which has left me wishing I had chosen something else.

It was like finally meeting someone you have long admired, only to discover they are not at all as you'd imagined, and not nearly as agreeable, or even admirable, as you'd hoped. That, at least, is how I felt as I ploughed through a reissue of The Blue Castle by LM Montgomery (Hesperus £8.99). As a girl I absolutely loved Anne Of Green Gables, the paperback's broken spine and ripped cover showing not only how often I read it but my sister before me too.

At first it's hard to see what the tale of an over-imaginative orphan adopted by a gentle Canadian farmer and his prosaic sister, in the late 1800s, could have to say to Scottish children three quarters of a century later. Yet small-town and rural Scotland in the 1960s was, in many ways, still out of step with the times. The emphasis on thrift, religion, good behaviour, education, and the closeness of community life – indeed the endless fascination of other people's doings – were scarcely different from Anne Shirley's time. How ironic, then, that a pivotal element of the novel, which felt like pure fiction, or something that certainly could never happen to us these days – namely the collapse of a bank – was lying in wait as a very real threat for all of us, some decades hence.

Lucy Maud Montgomery's myriad sequels to Anne Of Green Gables never appealed to me as the original did, and while I read them all, it was only once. In retrospect, though, these romantic and preachy tales pave the way for The Blue Castle, one of very few novels Montgomery wrote for adults. Hesperus describes The Blue Castle as an enchanting, delightful and witty classic. Maybe I am being humourless, but it seems to me more sour than spritely, more disillusioned than droll.

In essence it is pure fantasy, a good idea perhaps for a short story, but hard to carry off at novel length. The Canadian heroine, Valancy Stirling, mousy, dutiful and shamefully unmarried at 29, is desperately unhappy living under her tyrant mother's thumb. The other members of her self-important family are equally hard-hearted, prejudiced, unkind and ignorant. Eager not to underplay her plight, Montgomery lovingly recounts almost each insult and slight poor Valancy has suffered in her short life.

And a short life, it transpires, is all she can hope for. When a doctor tells her she has an incurable heart disease and will live another year at best, the worm turns. Valency stops being obedient, shrewishly answers her awful relations back and leaves home to look after a school friend with tuberculosis, who has been disgraced by having an illegitimate child, which subsequently died. While acting as nurse, Valency falls in love with a man her family considers a reprobate, with consequences that none could have foreseen.

In less intense hands, this could be a vivacious tale. The problem is, Montgomery seems to infuse it with heartfelt resentment. When even the pictures in Valancy's bedroom are a source of misery, you realise that there's something deeper at play in this plot than an enjoyable tale of rebellion. Given the period in which she was writing, Montgomery is modern minded and as feminist as Germaine Greer ever will be. But she is also morbid and girlishly fanciful, an overlush imagination too often undermining her daring political intelligence.

None of this would matter if The Blue Castle could be dismissed as a curio, a relic of a more innocent time. But what is disturbing is that in every chapter and character cameo, every dream and aspiration, there is an echo of Anne Of Green Gables. In that almost perfectly judged and genuinely comic, as well as tragic, work, Montgomery handled her material with the skill of a truly liberated artist. Here, by shedding the light and sympathetic style millions of children have adored, she loses much of her voice, and even more of her sparkle. How disappointing.