While the book is indeed good, it's pretty hard to do anything to improve that particular codebase because there are developers who are actively introducing more and more of the problematic patterns and practices even as i write this.
To them it isn't "legacy code" but just "code", while attempting to offer alternatives either earns you blank stares or expressing concerns about anything new causing inconsistencies with the old (which is a valid concern but also doesn't help when the supposedly consistent code is unusable).
To me it feels like it's also a social problem, not just a technical one and if your hands are essentially tied im that situation and you fail to get the other devs and managers onboard, then you'll simply have to either be very patient or let your legs do the work instead.
It's transformative in situations like this, it has a bunch of recipes for solving these kinds of problems.
While I don't use Java or C++, this book has probably been the most useful to me in working with larger bodies of code.