This might or might not be true of technical books as a group, but probably is untrue of textbooks, a great many of which are derivative, redundant with other textbooks, and sold mainly to the authors' students or at least students in the schools where the authors teach. I'm not claiming that's wrong, as I have accumulated several such textbooks that I thought at the time were pretty good, one of which I used in galley proof and helped correct. For most of them, though, the main royalty issue for the authors would have been former students selling them on as used books, something I understand some authors handle by issuing revisions every few years and requiring the current revision.
Biting the hand that feeds IT
