At a time when we yearn for one definitive figure for what the e-book market is worth, the Publishers Association offers us three different numbers from its annual yearbook: £16m, £13m and £20m.

£16m is the figure the PA has for "general consumer digital" sales, which include digital audiobooks, e-books,  downloads of all or part of books, subscriptions/access to online book publications, and any other wholly digital material delivered online or via CD-ROM.  Included in this is fiction sales £6m, non-fiction of £1m, and children's titles of £1m.

£20m is the figure above, but rounded-up to include the 30% of publishers who don't provide figures to the PA. The PA says this could be anything between £17m and £20m.

£13m is the figure for publishers e-book sales and downloads, with audiobook sales stripped out. This was up 570% on the previous year's number of £2m.

We can, of course, throw another number in the mix, since the PA's figures are based on publishers' invoiced prices, we might extrapolate that the e-book market is 30% to 50% ahead of the numbers quoted.

If we use the £20m figure, and imagine that e-books are sold at an average discount of 50%, then we can extrapolate that the consumer e-book market at retail prices was worth £30m in 2010. Of course in this world we have to imagine that most e-books are not sold at rock-bottom prices by Amazon.

At that value e-book sales represent about 2% of the overall consumer market (as measured by Nielsen BookScan).

The PA puts out a bigger number, saying that (at invoiced prices) digital sales represent 6% of overall publishers' sales, but that includes academic and professional sales, which are obviously well ahead. It too suggests that the consumer book market is worth 1% to 2% of the overall consumer book market.

Sales are growing fast, but as we've always said that did not begin to get meaningful until the last few months of 2010, and the first quarter of 2011. Already Hachette UK is saying its quarter one e-books sales were worth 5% of its total sales, and that the UK is now fast catching up with the US.

We are mid-way up the roller-coaster, but cannot yet see the high point.