Were e-book sales in the UK worth £105m in 2011? That was the figure implied by Hachette UK when it stated last week that its e-book sales of £21m amounted to a 20% share of the UK e-book market. Hachette added that its own e-book sales had grown by "nearly 500%".

We do not know if Hachette's figure was stated at invoiced or published prices, and whether it included audio-book downloads and/or app sales, but either way it seems unlikely that Hachette's own e-book growth will not have been reflected in the wider e-book market, meaning a second year of growth at 500%.

[Last year the Publishers Association's put a figure of £13m on e-book sales at invoiced prices suggesting that the total market - at retail prices - could be somewhere between £20m and £25m: this it said was up 570% on the previous year's number. A second way of measuring the size of the e-book market in the UK might be to look at the decline in print book sales, which fell £175m-£200m, with perhaps £100m-125m of that directly related to the impact of e-book sales.]

Whatever the absolute numbers, what this means is that consumer e-book sales are growing at a fairly consistent rate in the UK, and that rate appears to be much steeper than the rate seen in the US over an equivalent period. Since the Kindle launched in the US, in late 2007, trade e-book sales, as measured by the Association of American Publishers, have tended to at least double each year—2007 ($32m), 2008 ($53.5m), 2009 ($166.9m) and 2010 ($441.3m)—but they have never grown by as much as 500% in one 12-month window.

One reason we are growing at a quicker pace could be that we began with a more mature e-book infrastructure thanks to much of it having been imported from the States, and pent-up demand from readers hearing about the new technology but not being able to try it. Though Waterstones began selling e-books in September 2008, the Kindle did not arrive here until late 2009, two years after its US launch, and even then it took another ten months for the UK Kindle store to open.

It means that we are probably further along the track in terms of customer demand for e-books, but perhaps still a year to 18 months behind in customer conversion to e-reading. In other words the early adopters would have adopted earlier, but the stragglers will convert at the same pace as seen in other markets.

After three years of remarkable growth it now appears that the US market is flattening out—the AAP has yet to release its full-year 2011 estimate, but with e-book sales reaching $780m after 11 months, it seems likely that sales will have doubled, rather than "more than doubled".

In the UK the above analysis suggests that the slow-down might occur slightly sooner [Mike Shatzkin at IfBookThen suggested that one of the reasons the US market might be witnessing a slown-down in the rate of growth of e-books was that early adopters tend to read more, so when other readers convert the market still grows but at a lower rate than before].

In the UK another five-fold increase in e-book sales in 2012 would put them up to £500m and beyond, giving e-books a market share of perhaps 30% of the overall consumer book market, which was roughly what it reached in the US at its monthly peak over 2011.

There are three things to look out for, however. First, these numbers do not include sales of self-published writing, predominantly sold on Amazon for a song, but nevertheless a growing contingent of the overall e-book market. Second, we cannot yet factor in the impact of a Waterstones digital device coming on to the market: the launch of the Nook in the US clearly spurred growth over there, and might do the same in the UK. Third, we cannot yet begin to assess how much the arrival of cheaper tablets will have on either the illustrated book market, or children's book sales, but imagine what will happen to digital sales if those bits of the market that currently seem resistant to digital begin to move across . . .