I had my yearly “paperwork day” today. As part of that ordeal I was sorting the Cisco Press book sales reports and stumbled across e-book data for my MPLS VPN books. I can’t tell you how well the Safari access is doing (electronic subscriptions are bundled with numerous totally unrelated items into the “Others” category), but the reports have separate line items for PDF and Mobile (I assume that’s Kindle) edition. The sales of these editions are negligible compared to the “regular” sales.
Obviously even the highly technical audience is not interested in electronic books (or someone bought a single PDF copy that’s now enjoyed by the whole Internet) ... or you feel (like I do) that the reference books belong on the bookshelf.