"Work continues" is meaningless--and in the history of wargaming it is used all too often to mean "work is not actually taking place at the moment but we haven't necessarily given up on the project."
Hopefully it doesn't mean that, but the only evidence to the contrary I could offer is MMP's track record of not doing things like that, I suppose.
But if I may, "work continues" may be shorthand for "You guys really don't want to hear all the niggling and excruciatingly tedious details of what we have to deal with in scoping out what the next ASLRB would look like." I can imagine it could involve engagement with the printer, tradeoff between amount of paper used and cost, and how that relates to the font size and graphics used, maybe even how much the final product would weigh and how that relates to shipping, paper quality issues, longer-term issues relating to how to format the product for easier maintainability and cross-benefits with the electronic version, etc.
So, no, I imagine there could very well be a lot of irons in the fire that don't necessarily lend themselves to easy milestones like, "Done with Chapter C, now on to Chapter D". Now, you're right that glossing over all that does tend to lead one to think, "Well, what IS going on?", and in those cases, I tend to offer some level of detail without boring my customer silly: "We're looking at some formatting tradeoffs and how they relate to cost and maintainability down the road." That kind of thing is giving the customer a true story without making them sorry they asked.
Course, Perry may also just hate us and enjoy keeping us in the dark. I mean, let's not forget that.