Martin Kleppmann, the author behind Designing Data Intensive Applications, wrote about his experience as well, and it shows an interesting contrast with Resig's experience with digital publishing. As you can see in Martin's graph, ebook sales starting Sept 2014 were a _major_ part of his royalties due to it being available as "early release", and integration with the O'Reilly platform increased his exposure, and therefore royalties.
Its hard to guage accurately, but it seems O'Reilly + ebook sales contributed to about 2/3rds of his overall royalty returns, which is a pretty darn good result!
Of course, Kleppmann and Resig are writing about very different eras in terms of publishing, but I can't help but wonder if Resig would have a different experience if he was able to publish an equally relevant work in 2015 vs 2008.