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It is an iron rule that politicians are much more interesting when they are out of office. Ministers become frozen in the chilled embrace of government policy, obliged to toe the party line in speech and thought for fear of excommunication by Number 10 (or 11 - whichever Gordon Brown happens to be in at the time). In the current regime it would seem they also have reason to fear physical violence, too.

However free-thinking they may be at the start of their careers, ministers become worn down by the need to defend the Government's record, and by the absolute prohibition on admitting to past mistakes. So they are oh-so-dull to be around.

Even his detractors, and they exist, would not accuse David Willetts of the crime of dullness. It is a long time since he was infected by the virus of public office. In fact he is one of the few members of David Cameron's team with ministerial experience, yet his position in a future Conservative government is unclear. Unusually, he shadows a non-existent department, the late, lamented Department for Innovation, Universities and Skills. The man who casts no shadow is a well-worn literary trope: the shadow with no real man to call his own is a less familiar concept.

He has never been afraid to speak his minds (he has two of course). On occasion, that tendency has landed him in tepid water, as when he offended fellow Tories by questioning the social impact of grammar schools, despite the party's traditional reverence for selective institutions. Most of us hope he maintains that proclivity for intelligent commentary for as long as possible. Realistically, though, this is quite likely to be his last word as a free man for some time.

It is perhaps surprising that his long dark winter of opposition has not borne fruit of this kind earlier. But it was worth the wait. The Pinch, which emerges just before the intellectual blackout of the general election campaign descends upon us, is one of the most thoughtful and provocative books to emerge from a politician's processor in the past 20 years. This is as far from Norman Fowler's remaindered memoirs, or Alastair Campbell's diaries, as it is possible to go.

The central Willetts thesis can be quite simply stated. The baby boomer generation, now entering retirement - or thinking fondly about it - has done, on average, very well. They have benefited from rapidly rising house prices, and from final salary pension schemes, which disproportionately reward those who stay longest and earn most. The successor generations will not enjoy defined benefit schemes, as we know, and will find it very hard to get a toehold in the housing market. Even the recession has not brought prices down far: they are held up by constrained supply and a growing population.

This phenomenon might not be so worrying were it not for one other factor. Much of this boomer wealth has been spent. The UK's savings ratio has fallen to a dramatically low level, and our households are, on average, more highly indebted than even their American equivalents. So the generation to which I (and Willetts) belong "has converted a once-off ... surge in asset prices into higher incomes by borrowing against them (which) delivered a temporary boost in their living standards financed by a massive reduction in saving and imposed higher costs on the next generation, who have less to inherit".

But the journey is as important as the destination. Willetts writes with lucidity, elegance and wit. He has read extraordinarily widely. Indeed, the range of his sources is remarkable. The thought that a member of the Cabinet may be familiar with "the literature" of sociology, anthropology and public economics is faintly worrying. Surely some of the works he cites were not intended to be understood?

Along the way he unearths some fascinating data. Those Lynx deodorants that are supposed to make men irresistible to the opposite sex are largely bought by mothers who want to get their twentysomething sons out of the house at last. By the age of 45, it is unqualified men, and women with university degrees, who are most likely to be unmarried (and the two groups don't mix well). Students used to acquire 0.15 of a confidant per year of university study; now it is only 0.08. True friends are harder to find, it seems.

Does he have a solution to the central problem? Up to a point. He argues for a stronger sense of intergenerational solidarity, and for reknitting the up and down (the age range) strands of society. It is not a White Paper, and none the worse for that. But he convinced this reader, at least, that any future government is going to be wrestling for some time with the after-effects of the great pinch he describes.