> One lesson is that you have to be willing to walk away.

That's probably because I did not explain it well enough. I can tell you the lesson, because it is impact it had on me :-).

I was already willing to walk away and was about to. I found the book by chance, in something like a random HN discussion. Without the book I would have resigned without asking for a raise, I know it.

Then I got a few lessons about the way I should prepare for asking for the raise, and the big mistakes I should not make ("don't say this", which seems obvious when you read it, but which I would have done wrong).

Finally it helped me talk about salaries when I was interviewing for other jobs. Again, difficult to say if I would have ended up with a lower salary without the book, but it most definitely gave me the confidence to talk about compensation, and I think it's worth more than the $40 of the book.