It seems like starting work before a contract is in place (save maybe very high-level exploratory work, like a table of contents) is a recipe for disaster. They don't really owe you anything until there's a signed contract in place saying you do.

Now, it's possible that Apress is problematic in that they strongly encourage people to work despite there being no contract in place ("we'll figure it out", "you have our word", "legal is slow", etc), which then puts them in a better negotiating position when it is contract time because people have already done much of the work.

But I wonder what would have happened if he had said, "I'll do high-level planning, but I won't write a single paragraph of actual content until we have a firm, mutually-satisfying contract in place." Maybe it would have gotten him a firm date on when he would get his advance that they would stick to, plus deadlines in the contract that were actually in the future and other good things. Or maybe they would have still said, "no, just write it, we'll figure that out later", which would not look good