One lesson is that you have to be willing to walk away. Doesn't mean you can't negotiate at all otherwise. But you're in the strongest position when you legitimately can go "take it or leave it" if it comes to that.
And you also don't want to throw away a great opportunity because you didn't wring every last dollar out of it.
> 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.
You may enjoy 'Never Split the Difference' by an FBI negotiator. Interesting read for high stakes (kidnappers etc) stuff