Man Booker judging is famously hard work: 140-odd novels, most of which arrive in the spring and early summer, putting paid to any idea of weekends off, or holidays staring into space, or train-rides with the newspaper. Although the invitation to chair the panel this year was a hard thing to resist (and one piece of evidence among many about the high status of the prize), I said "yes" with a degree of caution. What would happen to the rest of my life? And what about all those poems I might want to read?

In fact the whole business has been so enjoyable, I've hardly noticed the hard-work element. This is partly because my fellow judges have been very good company (Deborah Bull, Rosie Blau, Tom Sutcliffe and Frances Wilson), and partly because the books have been so . . . well, so interesting, even when not so good. Too many publishers publish too much. Not nearly enough novels get the editing they need. Some novels are so clearly manifestations of distress, they might be better described as "a frieze of misery" (in Larkin's phrase) than a work of fiction. But for all this, the second-rate books all felt worth reading (as a way of range-finding) and also valuable (as a way of fertilising the ground for those other and better books that rise above them).

And those better books are, of course, the main point. I should say of our 140-odd, about 100 completely repaid serious attention – and in order to discover these, we have met for supper at regular intervals since last Christmas, discarding or preserving titles as we went. This had the double advantage of allowing us to get to know one another's tastes and of making our task feel manageable. By the summer, we'd got down to fewer than 50. Then we met to decide our longlist of 13 – which emerged pretty easily (more than half the titles were unanimous decisions; the rest were chosen on a points system). This week we've whittled the 13 down to six: Parrot and Olivier in America by Peter Carey, Room by Emma Donoghue, In a Strange Room by Damon Galgut, The Finkler Question by Howard Jacobson, The Long Song by Andrea Levy, and C by Tom McCarthy.

Inevitably, the early press responses tend to notice who's not included. With the longlist it was "What, no Amis or McEwan or Rushdie?"; with the shortlist it's "What, no Mitchell or Tsiolkas or Tremain?" That said, the general reaction to our choices has been pretty enthusiastic; in fact Booker-watchers have assured us that our longlist has sold better than any for several years. Although we didn't set out to be "popular", we were pleased to be told this. Isn't one of the purposes of the prize to get people reading what they otherwise might not?

So what did we set out to be, if not popular? Open-minded. That's to say, we had no preconceived wish to reward particular styles, or genres, or themes, or individuals. We wanted to find books that seemed "the best", and to base our decisions on what we found in front of our eyes. And how would I characterise this "best"? By all the means that sound familiar, but make their material strange. Good writing (adventurous as well as scrupulous). Imaginative daring (confirming what we feel might be true about the world, by means that are surprising). Historical sense (an ability to see the present in the past, and vice versa, so as to illuminate both).

If nothing else, our search for these things has allowed us to make a list full of variety. It's conventional for judges to make such a point – partly to emphasise the range of work on offer at any particular time, and also perhaps as a way of reassuring themselves about the breadth of their taste. But this year the diversity is genuinely striking – in terms of background (Carey the Australian who lives in New York; Donoghue from Ireland now in Canada; Galgut a South African), as well as historical moment (McCarthy ranging through the early years of the 20th century; Levy evoking a slave plantation in Jamaica; Jacobson in contemporary England), and subject (Jewishness, democracy, kidnap, subjugation, human communication – and the lack of it).

There's a remarkable range of tones in the books, too – from Galgut's astringent severity to Jacobson's often fairly broad comedy. In the public response so far, it's the comedy that has attracted most comment, as if it represents a departure for "Booker books" to have a sense of humour. Maybe it does, in which case I suppose our choices might connect with other ways in which comedy has joined the mainstream (standup, "youth culture" and so on).

Having said that, the kinds of smile provoked by these books are so different from one another that it gives a false impression to speak of them as predominantly humorous. Even the most laugh-aloud funny (Jacobson) is really laughter in the dark, and the wit that appears elsewhere is often very bleak indeed – in Room, for instance, it's a brittle but vital defence against terrible circumstances, and in the Carey and McCarthy novels it's more a proof of exuberance and imaginative defiance than of finding things risible. All this leaves me thinking – a month away from having to make a final decision about the winner – that what characterises our list is the difficulty of fitting it into a neat category. Which feels like a fitting tribute to pay to a form that has always thrived on notions of surprise and unpredictability. At the same time, it creates a different and more local kind of suspense. At the end of our shortlist meeting, we piled our six choices in the middle of the table at which we were sitting, and asked one another who we thought might win. None of us could be certain. The books are all outstanding in their own way. It's very good to have the chance to reread them between now and 12 October.

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