The nation's reading in the first half of the year has been dominated by Dan Brown and diet books, with a peppering of literary and break-out fiction, sales figures reveal.

The 5:2 Fast Diet, a regime of intermittent fasting during which dieters eat normally five days a week and then for two days cut calorie intake by three quarters, has sold 402,887 copies – making it the No 2 bestseller.

The lifestyle and recipe book, penned by medical journalist Michael Mosely and fashion writer Mimi Spencer, taps into the nation's obsession with staying healthy and living longer; the pair even claim their diet can ward off Alzheimer's disease.

In fiction, the break-out hit is The Hundred-Year-Old Man Who Climbed Out of the Window and Disappeared by Swedish writer and media entrepreneur Jonas Jonasson, published by the small indie press Hesperus. First out in July last year, it has found an audience through word-of-mouth recommendations to reach bestseller status, and charts at No 15 with UK sales of 128,061 during the first half of 2013.

Philip Jones, editor of book industry magazine the Bookseller, said: "The Hundred-Year-Old Man … was a quiet sensation that got supercharged by its inclusion in the Sony 20p ebook promotion. It and Life of Pi were the biggest sellers across print and e in the fourth quarter of 2012 and the first quarter of 2013, with digital outpacing even the strong print sales."

He added: "It's a game-changer for Hesperus, as such hits always are for small presses."

Life of Pi, first published in 2001, was reissued to coincide with the release last year of Ang Lee's film adaptation of the book, and charts at No 7, with sales of 149,373 copies.

Notable by its absence from the bestseller list is any serious non-fiction title such as a prominent biography, significant science book or weighty history tome. Robert Macfarlane's nature book The Old Ways (A Journey on Foot) has sold well but misses out on a spot in the Top 20.

"Publishers traditionally keep their big non-fiction books back until the end of the year, so they can benefit from the huge gift market that coalesces around Christmas, or any television adaptations," said Jones. "Publishers are quite wedded to these schedules."

As a counterpoint to extreme diet regimes, a more traditional low-calorie recipe book, Hairy Dieters: How to Love Food and Lose Weight, the tie-in to Dave Myers' and Si King's BBC TV series of the same name, is at No 4, having shifted 263,055 copies.

The fiction entries are led by Dan Brown's Inferno, still at No 1. It has sold more than 500,000 copies in hardback since 14 May 14, having swept into the top spot in its first week of release with sales of 228,961 copies. Although Brown tops the list, overall sales are lower than the first-week sales of his previous novel The Lost Symbol, which sold more than 550,000 during publication week in 2009.

Rachel Joyce's debut novel The Unlikely Pilgrimage of Harold Fry, which was longlisted for the Booker prize, has found a broad audience for a first-time writer, selling 236,163 copies. It tops the fiction bestseller chart (which excludes '"genre" fiction such as crime and thrillers), and is at No 5 in the overall list. The Guardian's review of Joyce's book, about a man who sets out on a long walk after finding that a former colleague is seriously ill, said it delivers "unexpectedly savage emotional blows … tempered with a sense of quiet celebration".

Official book sales figures are compiled by Nielsen BookScan.

The UK Top 20 bestsellers January-June 2013

1. Inferno (Robert Langdon Book 4) by Dan Brown (Bantam) 501,384

2. The Fast Diet by Mimi Spencer & Michael Mosley (Short Books) 402,887

3. Gone Girl by Gillian Flynn (Phoenix 391,923)

4. The Hairy Dieters (How to Love Food and Lose Weight) by Dave Myers & Si King (Weidenfeld & Nicolson) 263,055

5. The Unlikely Pilgrimage of Harold Fry by Rachel Joyce (Black Swan) 236,163

6. Entwined with You (A Crossfire Novel) by Sylvia Day (Penguin) 167,495

7. Life of Pi by Yann Martel (Canongate) 149,373

8. A Wanted Man (Jack Reacher 17) by Lee Child (Bantam) 140,662

9. The Hobbit by JRR Tolkien (HarperCollins) 135,581

10. A Street Cat Named Bob (How One Man and His Cat Found Hope on the Streets) by James Bowen (Hodder) 135,534

11. The Woman Who Went to Bed for a Year by Sue Townsend (Penguin) 135,349

12. Diary of a Wimpy Kid: The Third Wheel by Jeff Kinney (Puffin) 132,466

13. Horrid Henry's Guide to Perfect Parents by Francesca Simon (Orion Children's Books) 131,515

14. Kill Me If You Can by James Patterson (Arrow) 131,083

15. The Hundred-Year-Old Man Who Climbed Out of the Window and Disappeared by Jonas Jonasson (Hesperus Press) 128,061

16. Best Book Day Ever! (So Far) by Tom Gates and Liz Pichon (Scholastic) 125,364

17. The Mystery of Mercy Close by Marian Keyes (Penguin) 124,566

18. The Fault in Our Stars by John Green (Penguin) 117,718

19. I've Got Your Number by Sophie Kinsella (Black Swan) 115,764

20. Eloise by Judy Finnigan (Sphere) 114,844