I have hit the halfway point. Four months in, 75 books read, 75 left to go. Judging the Man Booker Prize, to paraphrase Bette Davis, is not for sissies. I did the Orange Prize last year (about 50 books for each judge), and it ate up all my free time: during every bus journey, every moment sitting waiting for a film to start, every interval of whatever play I was reviewing, I'd whip out a book and cram a few more pages in.

This is different. We had 50 books to read in the first three months, and a book every other day is fine. Then publishers submitted more. A lot more. My reading speed had to double overnight: between March and July, I will have read the final 100 books in 100 days. You get ahead sometimes (a couple of short books in a row), and then a 900-page monster lurks behind them on the shelf, gobbling up the spare day and spitting out its bones. It's like running on sand, but less healthy.

It robs you of the chance to talk about books, too: I'm not allowed to tell you which books have been submitted for the prize, so I can't discuss them with anyone but my fellow judges. And I don't have time to read the books everyone else is talking about (what's the plot twist in Gone Girl? I hate not knowing). Not that this has stopped publishers sending over other books with a jaunty note, suggesting it might make a nice break from the Booker reading. I'm tempted to write back, telling them that I'm already using that time to sleep.

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But on the plus side, I know a lot more than I did about world history. It's fair to say that the Mantel effect is still in play; I have never read so much historical fiction in my life. It isn't quite the majority of entries, but it's not far off. And it's global: historical stories set from the top of the world right the way down to the Antipodes, via Scotland, Ireland, France, Italy, Russia, and America, among others.

There's very little future fiction, which I hope is still the term used to describe things set in the future but without aliens/spaceships. It's a long time since The Handmaid's Tale and Never Let Me Go. At the moment, writers – and readers – certainly seem more interested in imagining the past than the future.

Meanwhile, Africa, Asia and the Middle East are proving a fertile ground for contemporary fiction. And the Booker is a broad church. We've been sent thrillers, love stories, family sagas, war novels, spy novels, detective novels and sequels (another consequence of the second Mantel victory?). There have been a lot of quirky child narrators, which suggests to me that The Curious Incident of the Dog in the Night Time continues to cast a long shadow, quite apart from its recent Olivier success.

The question I am most frequently asked about prize judging is, "How do you read all those books?" In close second place comes, "Where the hell do you put them?" to which the answer is that my flat could currently host a literary jenga tournament, no problem. But the question that really plagues writers is this: how do you compare the relative merits of novels? How do you choose between a great story and great prose?

And after extensive research, I have an answer. I don't choose between them, because the best books, for me, must have both. Beautiful writing isn't enough unless your story makes me want to turn the page and find out what happened next. I'm not judging based on phrases alone. I want a whole book to get lost in: characters I care about (which isn't the same as liking them), and a story which resolves itself perfectly. Gorgeous sentences trapped in a dull book won't do.

A well-paced plot with workmanlike prose won't do, either, because they just don't stick in my memory. Which doesn't mean I don't enjoy them – I probably read every Agatha Christie 10 times as a teenager, and I still can't remember who did it, except in The Murder of Roger Ackroyd. When I scan my notebook – I'm making notes on each novel as I finish it – I find there are books I have read which I can barely recall. If it doesn't stick in my memory for a few months, even allowing for the dozens of other books which overlie it in my brain, then it doesn't deserve to win a prize.

Overall, my mid-term report on contemporary British and Commonwealth fiction is that it is in excellent health. Yes, there are some limping gazelles, but there are wonderful books being published every week, from commercial thrillers to high-end philosophical conundrums. Only 74 left to go.

The winner of the Man Booker Prize will be announced on 15 October