Turning a conference proceedings into a book chapter. Is it self plagiarism? I've recently sent some of my research to a local conference entitled "XIX Simpósio Brasileiro de Sensoriamento Remoto". After the conference, I started getting contacted by some publishers asking me to publish the work I sent to the conference in the form of a book chapters and another one even invited me to publish as a paper itself in a journal. None of them required me to edit the work in any way.

I'm aware that academics get scammed this way (I managed to identify some myself), but this particular invitation seemed okay. The editor was very honest about the publisher status ("We're not big", "We aim to achieve this and that in X months", etc...) and I have colleagues that have published with them and nothing went wrong (meaning publication really happened, the e-book is available for download and all). 

Then, I received an e-mail by the conference staff alerting all participants to the risks of plagiarism if one is to accept these publishers' offers. They argumented that the conference proceedings are open access, they have a ISBN and used this as to validate their statements. Additionally, they said they would be taking legal measures. Honestly it seemed more like a power play than anything.

Normally, I would agree with the e-mail, face this as self plagiarism and don't even consider this as an option, but a fellow colleague pointed out that conference proceedings doesn't count as a publication because it has no DOI nor ISSN (both entries are used on my country to score for PhDs and funding, but not ISBN). In his mind, as far as academy goes, my work is still "new" and there would be nothing wrong in publishing it, especially considering nobody cites conference proceedings.

I understand that it is normal to use the conference proceedings as a type of feedback while you're developing your research, but I published a substancial amount of my results on this paper and I don't really know if I could only cite it on this book chapter invitation I received, otherwise it would be nothing left to talk about.  

So, what I want to understand is: what are the rules to consider something properly published, and therefore, accountable for self plagiarism? Is there a way to publish this book chapter in a unethical way? For example, including that this chapter is a reissue of conference yadda yadda would do?
You have a few misconceptions. Having an ISBN is no guarantee of legitimacy, honesty, or quality as anyone can purchase such. In bulk they are very cheap. I currently have several unassigned ISBNs myself, intended for future work. 

Second, if your paper has been submitted to a conference and appears in the proceedings then it is "published" and so subsequent uses are not original. 

However, self plagiarism only occurs if you reuse some work without citing it. If you still hold copyright, as is likely, you can reuse the thoughts and many of the actual words as long as you are careful. Cite the original conference work as appropriate and formally quote from it as necessary. Perhaps you gave up copyright already, but I think that is unlikely in this case and you would know anyway if you had signed a transfer. And it isn't the ISBN on the original that makes it self plagiarism. It is copying without citing. 

Cite your own work the way you would that of another author. 

The main purpose of avoiding self plagiarism is to permit a future reader to trace the full context of the ideas in the new work by examining the old with its full context, including all references you cited there. Future researchers want a full record. Ordinary plagiarism includes this as well as other concerns, of course. 

But some of what the publishers seem to be saying seems a bit of a scam. I can't say without more information, but it has that feel. Legitimate publishers also have "page charges". Try to assess the legitimacy of the publisher using other information before you agree.