When my C book was first made available as a freely downloadable pdf, in 2005, there were between 19k to 37k downloads in the first week. The monthly download rate remained stable at around 1k per month for several years, and now floats around 100 per month.
I was hoping to have many more downloads for my Evidence-based software engineering book. The pdf became available last year on November 8th, and there were around 10k downloads in the first week. Then a link to my blog post announcing the availability of the book was posted to news.ycombinator. That generated quarter million downloads of the pdf, with an end-of-month figure of 275,309 plus 16,135 for the mobile friendly version.
The initial release did not include a mobile friendly version. After a half-a-dozen or so requests in various forums, I quickly worked up a mobile friendly pdf (i.e., the line length was reduced to be visually readable on a mobile phone, or at least on my 7-year-old phone which is smaller than most).
In May a link to the book’s webpage was posted on news.ycombinator. This generated 125k+ downloads, and the top-rated comment was that this was effectively a duplicate of the November post.
The plot below shows the number of pdf downloads for A4 and mobile formats, along with the number of kilo-bytes downloaded, for the 6-months since the initial release (code+data):
On average, there are five A4 downloads per mobile download (excluding November because of the later arrival of a mobile friendly version).
A download is rarely a complete copy (which is 23Mbyte), with the 6-month average being 1.7M for A4 and 2.5M for mobile. I have no idea of the reason for this difference.
The bytes per download is lower in the months when the ycombinator activity occurred. Is this because the ycombinator crowd tend to skim content (based on some of the comments, I suspect that many comments never read further than the cover)?
Copies of the pdf were made available on other sites, but based on the data I have seen, the downloads were not more than a few thousand.
I have not had any traffic spikes caused by non-English language interest. The C book experienced a ‘China’ spike, and I emailed the author of the blog post that caused it, to notify him of the Evidence-based book; he kindly posted an article on the book, but this did not generate a noticeable spike.
I’m confident that eventually a person in China/Russia/India/etc, with tens of thousands of followers, will post a link and there will be a noticeable download spike from that region.
What was the impact of content delivery networks and ISP caching? I have no idea. Pointers to write-ups on the topic welcome.