Taking animals seriously

I've started making an effort to read more serious literature on animal rights, interests, ethics, or whatever you'd call it. At the moment I'm reading David De Grazia, Taking Animals Serious and I'm struggling because of its high level. It requires a background on ethics and an understanding of how one constructs ethical theories. Admittedly, he says one can skip the more techinical part on ethical theory, but personally, I want to gain a solid grounding for my arguments on animal ethics. Yet, one thing keeps coming into my mind, surely the only important thing to consider, is that animals experience pain and suffering (physical and emotional) and it is wrong to cause any being (human or non) unnecessary suffering. Of course, what is "necessary" could be debated, but most people don't even get this far in their reasoning.