'Boundaries After a Pathological Relationship' by Adelyn Birch is good.
Also Natalie Lue's Baggage Reclaim website has lots of information on boundaries and also all sorts of good relationship information in general. As a people-pleaser you will finds lots of useful articles there - there's a search box you can use to search for info on boundaries, for example.
The main thing with boundaries is that you don't lecture and prescribe to people in an attempt to make them abide by your boundary. You maintain a boundary by your own actions. So if someone is acting in a way that is making you uncomfortable, you express that clearly, state the way they can rectify the situation, and if they are unable to behave in a way you are comfortable with, you remove yourself from the situation or the person.
Sometimes you don't even have to express the boundary with the other person at all. For instance, if you have a 'no dick pics' boundary, and a guy sends you a dick pic, you can just block him, no need to explain why because he's a stranger.
Whereas if someone lies to you, you may feel it's better to state, 'I cannot be in a relationship with a liar, so please don't lie to me again.' Then if they do lie again, you end the relationship.
Some things you won't want to give a second chance to - like infidelity or any form of abuse.
It may also help you to think of 'being boundaried' rather than 'setting boundaries'. Being boundaried is about knowing where you end and another person begins, and means you understand what you are responsible for, and what belongs to another person. People-pleasers often neglect their own needs in catering to another person's desires, then find themselves feeling resentful.
Whereas 'setting boundaries' implies that lecturing and persuading of another person in an attempt to make them respect your boundary. In reality, the only person who has to respect your boundaries is you.