Nielsen BookScan is now articulating what many have previously thought: that e-book sales are depressing print sales.

According to new sales data coming from the US and UK first quarter sales in the US are down a full 9% in volume on last year, from 178m in the first quarter of 2010 to just over 162m this year. In the UK volume sales dropped slightly less - by 5.5% to 44.5m. Elsewhere, in Denmark, Italy and Spain for instance, there was in fact a small degree of book sales growth in the first quarter - presumably as a consequence of the slower rate of adoption of digital.

Speaking at a meeting of European Booksellers Federation held before this year's London Book Fair, Jonathan Nowell, president of Nielsen Book, pointed to a particularly marked drop in fiction sales on both sides of the Atlantic, with the UK down 9.8% on 2010 figures in the first quarter, and the US down a massive 19.3%. According to Nielsen figures, fiction is the dominant e-book genre, taking a share of 70% compared with its print share of 30%.

Said Nowell: “We can surmise that e-book sales may be affecting fiction more than other genres, and responsible for the steep downturns in the US and UK."

Of course, until Nielsen develops an e-book panel--still “months rather than years” away--it can tell us how much print is down, but not how much digital is up. This makes analysis difficult. As Nowell acknowledges there are other factors depressing sales: the problems at Borders, a reduction in frontlist space at B&N, the recession, unfavourable comparisons after the Steph and Stieg years.

But digital is now a tangible part of our businesss: the measurement of that activity cannot come soon enough.