I looked for resources a few years ago and found a post by a software engineer talking about "Fearless Salary Negotiation" [1]. The engineer said: "it's a small book, cost me around 40 bucks which were immediately compensated by the fact that I managed to negotiate a higher salary".

I wanted to ask for a raise: I had been working in that startup for 4 years and had never had a raise. I thought "if that book helps me get a raise of a few bucks per month, that will pay for it".

It didn't go as planned: I followed the instructions in the book, my boss spent an hour bullshitting me and I didn't get a raise. So I sent my resignation the next day. The boss called me back, and I got a substantial raise and a bonus (to compensate for the shitty salary I had been having before, it was not a miracle).

All that to say that this book did not make me a pro negotiator. But it made me understand how salary negotiation works (reading it, I felt like a child: it only says common sense stuff, but I had been doing everything wrong my whole life). And it gave me the confidence to actually ask for a raise.

Totally worth it.