As a student touring Algeria in the 1970s, I learnt a useful phrase from the dog-eared Michelin guidebook (in French) on which my travelling companion and I relied. “Francis,” I had asked, why does the expression “très curieux” recur like a catchphrase in Michelin, to describe all sorts of things and places? Why should so much in Algeria be 'very curious'?”

“Don't take the words at face-value,” Francis said. “It's a peculiar euphemism in Michelin. It means 'Don't go out of your way to see this'.”

And he was right. A particularly nondescript oasis, a flyblown casbah, some unusually shaped rocks in the Hoggar mountains... all were “très curieux”, and none proved worth visiting.

So we learnt the code. But what is it with guidebooks and their inability simply to spit things out? A must-see is praised lavishly; a should-see is praised generously; but a don't-bother is praised faintly - or, more often, left out altogether. It is as though the very act of publishing a guidebook to a country is to be taken as a kind of homage.

But once you've bought the guidebook, you're probably going anyway. And once you're going, one of the things you most want to know as you plan a too-short visit with too many places to visit, is what to leave out.

Baedeker's Great Britain (1887) points the way: “Oxford is on the whole more attractive than Cambridge to the ordinary visitor; and the traveller is therefore recommended to visit Cambridge first or to omit it altogether if he cannot visit both.” I may - do - disagree with that, but let's hear it for any travel guide prepared to put an opinion bluntly on the line.

The buildings (for instance) at Machu Picchu would be unmemorable in any other location. Manchester is horribly overrated unless you're an inveterate clubber; there's little to see. Barcelona is dangerous, and can be dispatched in a weekend: there are about five things to do. The Paris Metro at night is filthy and scary. The Camargue is frankly dismal. Newly laissez faire urban Russia is like Brazil without the charm.

As in the criminal courts, guide books - having counselled for the defence of each foreign attraction - should offer a 1,000-word window to counsel for the prosecution. Not just in the particulars but the generality, I want to read the tourist case against France; the horror of Dutch catering; the rudeness of Israel; the honest reason nobody goes to Germany except for business or family; the indefinable nastiness beneath the surface in Brazil; the deep gloom of Singapore; or Malaysia's curious failure to cohere.

And if you're contemplating a touristical walk from Westminster Abbey over Westminster Bridge, I want you to be cautioned to blinker your eyes, block your ears and hold your nose from Big Ben until you're well on to the bridge. For around Embankment steps a ghastly scene awaits.

To the screech of unlicensed buskers and the sickly smell of sugar-coated peanuts, low-grade hot dogs and onion frying in old cooking oil, you must push your way past British white trash buying football mementos, fat Bengali families buying milkshakes for innumerable children, massive-bottomed American teenagers buying plastic bobbies' helmets, and dodgy-looking Iranians selling tickets for this or that cheap ride, while mobs of Japanese tourists video each other in a mindless frenzy.

Why don't guidebooks warn us about this sort of thing?