Aha, thanks to @CraigM for the link, I think this answers my question
In our professional publishing program for developers and system administrators, we're most likely to publish books on topics with these characteristics:
- Covers standards-based technology (or is so widely used as to be a de-facto standard).
- Shows grassroots adoption and a community surrounding it.
- Has hard problems for users that can be solved by better documentation.
- Is not already covered by a lot of existing books.
- Has an author with real expertise.
- Covers new technologies that might not be on most people's radar but are on the brink of wide adoption or importance.
We're NOT looking for:
- Books that overlap too heavily with our existing books.
- Books on proprietary technologies that don't have a huge user base.
- Books on miniscule (i.e., personal or nascent) products, even if they are open source.
- Books on topics that have dismal sales despite quality books being available. (If you're addressing a topic where good books have sold dismally in the past (for instance, LISP, LaTeX, or Web-based training), you have a much higher threshold to clear with your proposal. Convince us why there is a revival of interest in your topic, or why your approach to a deadly topic will provoke interest nonetheless.)
- Books that have been rejected by other publishers, in most cases.
A Delphi book would certainly not hit on some of those criteria, but others it would. An author would need to make a solid case for publishing the book while Dr. Bob and Marco Cantu are alive and furthermore, would need to convince a lot of people that Delphi (or at least Object Pascal) is accessible and innovative.
The book wouldn't be a method of boosting popularity, it should arise with the popularity.
It is apparent though, that some other technologies have made this case successfully.