Don't know if you're still following this discussion Peter, but even in the past "making money" should not have been a primary incentive for writing a textbook.
I remember at DCC about 15 years ago Alistair Moffat did a little session on "why you should write a textbook" or something like that - I remember asking him "isn't it obvious?" but apparently it isn't obvious to a lot of people.
Frankly, the reasons for writing a textbook should be (either in the past or now) primarily to get your name out there as someone who understands the field at a level beyond what people who can't write a comprehensive book can do. There are big career benefits to this, and frankly much more valuable than any royalties you'd receive. In compression in particular, think about how valuable it makes you as an expert witness to be able to be "the person who wrote this widely read book on data compression". Hell, I've only written chapters in edited volumes on compression, and I've made some good consulting money off that.
I would hope that anyone who has seriously contemplated writing textbooks would realize and understand this - the side benefits of publishing a textbook are far greater than anything you get off of the obvious (and usually small) income stream. Rules are probably different for intro-level books (CS1/CS2 level - Nell Dale probably makes a decent amount of royalty income), but that's a very competitive market, and not one a data compression book is going to play in.