Michael Palin's new novel contains three jokes – one about British Gas, one about phones and one about bums – and that is all.

This book may be written by a comedy legend but it is not a comic novel; it is a light but serious, readable thriller with lots of momentum and pace provided mostly by its rather good title, The Truth.

The truth that the main character, Keith Mabbut, is looking for is mainly the truth about himself. He is a middle-aged man who has an unsatisfying job (writing the history of the Sullom Voe oil terminal), unsatisfying relationships and, worst of all, an unsatisfying future doing more of the same. He says to himself: "Before it is too late, I must correct the course of my life."

At first, Keith thinks this course correction lies in writing fiction, and in particular an epic set at the dawn of time that his agent keeps dismissing as "the science fiction thing". At the same time, he gets an offer to write a biography of Hamish Melville, an enigmatic humanitarian who travels the world highlighting the plight of native people being bullied by big industry.

It is as Keith decides which he should do – the science fiction thing or the biography – that we get at one of the book's most interesting strands, one of the beams that hold the novel up: does truth lie in non-fiction or fiction, and isn't non-fiction a kind of fiction too? At one point, Keith ask his girlfriend Tess which she prefers. "Oh, fiction every time," she says. "I hate facts. Facts are just facts. They don't amount to a row of beans. If you want the truth, read Jane Austen."

Slowly, as he digs into Melville's life story, Mabbut discovers this for himself. First, he breaks through Melville's public image to what he thinks is the truth, then he has to break through that too, constantly in search of the facts. To make matters worse, he also discovers that what he thought he knew about his friends, his family and his colleagues is also not so certain.

These plot twists, some more spottable than others, give The Truth much of its pace, but also make it wobble when we are expected to believe that a man who has won awards for investigative journalism wouldn't rumble some of the secrets. For example, would he really not have checked out the background of his publishers by, say, putting their name into Google?

Another issue is that the minor characters at the centre of the twists feel rather neglected by the middle of the book; they are Son of, or Daughter of, or Ex-wife of, rather than chunky, real characters that feel as if they go off and live away from the pages. As Mark Haddon proved in his recent novel The Red House, you can make every character in a large cast, even those with only one or two lines, feel substantial and likely and recognisable.

What prevents this problem with character crashing Palin's book is the fact Mabbut is a sympathetic man who will make the internal organs of some middle-aged readers twitch with recognition. Palin also has a light, smooth writing style, particularly in the sections set in Shetland and India, which probably isn't surprising from a man who has travelled to, and thought so hard about, so many countries.

And even though there is a slight feeling in the first few pages of the punchline that never comes, The Truth is a contemplative book. Palin has carefully considered how the end of relationships make people feel, and what middle-age can make some men do; but it is the central struggle between fiction and non-fiction, truth and lies, that provides most of the novel's energy and pull. One of the characters even suggests that too many good people spend their time writing fiction and it's a witty idea: that a writer like Palin would write a story about characters who can't see the point of writing stories. Maybe it is the book's fourth – and best – joke.

Michael Palin is at the Edinburgh International Book Festival on Monday at 6.30pm. Go to www.edbookfest.co.uk or call 0845 783 5888

The Truth

Michael Palin

Weidenfeld & Nicolson, £18.99