Bestselling biographies by Bill Clinton and Jordan helped boost growth at Ottakar's, the bookseller said yesterday, but could not fully match the surge in turnover last year when the fifth Harry Potter volume was published.

In a trading update, Ottakar's said like-for-like sales for the 23 weeks to July 10 increased 4.1%. Including the acquisition of Hammicks book shops made in April last year, sales increased by 19.9%. Although sales slowed from the last update, in June, Rhys Williams, an analyst at Seymour Pierce, said: "We believe that it is a solid performance since its comparative figures include the impact from Harry Potter." The shares closed down 6p at 386p.

Ottakar's was started in 1987 by its current managing director, James Heneage, and acquired branches of Thin's Booksellers in 2002 and 24 branches of Hammicks last year, giving it a total of 120 stores countrywide.

The outlets bought from Hammicks increased their sales by 3.2% for the 23 weeks to July 10. "Gross margins at the Hammicks stores have improved by 5%," said Edward Knighton, the finance director. Margins have improved due to improved buying power and product mix.

Ottakar's has no plans to open stores in big towns. "We take the view that there is less competition in smaller towns and can make good money by offering a decent book retailer," said Mr Knighton.

Philip Dunne, the company chairman, said gross margins for the year to date were "in line with expectations" but it was "too early to anticipate the outcome of our full year".

The threat to Ottakar's business model from internet booksellers such as Amazon also appears to have levelled off.

"Our model of selling a wide range of books with staff that can make informed recommendations is one way of dominating the mail order book business," he said.