Gwyn Headley has no head for heights. This is unfortunate as he is besotted with barking-mad buildings, those phantasms that spatter the otherwise respectable British countryside more thickly than any other.

His pursuit of folly takes him to every corner of the country - and indeed Europe and America - tracking buildings that were frequently built as ruins in the first place, and have become startlingly more authentically ruined ever since.

Typically, these mouldering wrecks, castles, abbeys, towers and obelisks, held together by ivy and graffiti, are to be found perched on beetling crags, on the rim of treacherous ravines or clinging to the crumbling edge of sea cliffs. Reading between the lines of his guide to some 2,000 British follies, you can clearly detect a lurch of the Headley stomach.

There is a stone oak in a Scottish forest, a baffling memorial to nothing in local memory, which he can hardly speak of without turning green. It's on top of a hill, of course, but also surrounded by unmarked and unfenced coal and iron-working shafts, and he approached it half blinded by sleet. "Not a good place to be," he says, wanly, "not a good place at all."

The guide is studded with little lists: of follies to stay in, pyramids, ancient follies. One list is headed, bleakly, Vertigo. The entry for White Nancy in Cheshire reads: "We have reached an age in our lives when we can quite justifiably stop to admire the view when climbing a particularly steep hill. Kerridge Hill affords many such opportunities."

Headley met his first folly as a child. His father explained that the tower in a forest was for nothing, just to give pleasure to the viewer. The boy was hooked for life.

The book, written with Wim Meulenkamp, co-founder of the Folly Fellowship, is a complete rewrite of their earlier guide. Since then, Headley has written a guide to American follies, and Meulenkamp a book that wrenches credulity: a guide to Dutch and Belgian follies. Hundreds of follies are included such as Headley's favourite new follies, the shell grotto at Leeds Castle in Kent - for some reason shell grottoes are the only feminine branch of an almost exclusively masculine profession - and a wonderfully mad garden in Yorkshire, the Forbidden Corner, which includes a mailed fist clutching a severed leg because, the owner explained, "it cost an arm and a leg".

Despite providing a six-figure map reference for each folly (but bear in mind his rant in the introduction about the number of follies incarcerated on private land), he still thinks they should best be tumbled upon by accident, preferable in a dense fog, and the questions they pose should tick in the brain for years.

"It's really shocking how few true follies there are - most have some dismally sensible explanation."

One of his favourite folly builders was Mad Jack Fuller, who produced a remarkable clump around his 18th-century estate at Brightling in East Sussex. He was both mad and kind - renowned for never turning a poor man from his door. He gave work and money instead of scraps from his table.

You can hardly miss the pyramid: it is plonked, like a hippopotamus in a hen run, in the sheep-nibbled graveyard around the Norman church in the centre of the village.

According to legend, Mad Jack was entombed sitting at his favourite dining table, with a chicken and a bottle of port before him: alas, when the vault was opened for repairs he was found to be lying down in the conventional manner.

The lack of a pub in the village is another part of the legend. Headley can't find a scrap of of evidence for the story, but has elected to believe it anyway: the vicar allowed Fuller to build his extraordinary tomb on condition he closed the village pub and opened a new one a mile away where it would be less of a distraction to his parishioners.

The Sugar Loaf is a mile across the fields, or two miles by winding roads. It has a superb view of Fuller's estate. Fuller boasted at his dinner table of his view, and wagered that he could see the spire of Dallington church, some four miles away.

The next morning, with the benefit of daylight and a clearer head, he recalled that Dallington was in a hollow, and its church scarcely visible from 100 yards away, never mind his front doorstep. So, in 24 hours, he built a sham Dallington steeple, and won his bet.

• Follies, Grottoes and Garden Buildings, by Gwyn Headley and Wim Meulenkamp, is published by Aurum Press at £20.