Currently, I'm reading Gary Steiner's Anthropocentrism and Its Discontents, a look at the West's historical understanding of the moral status of animals. One point that jumps out at me as I'm reading is that, despite my particular interest in the topic, it's not a terribly important book. That is, if someone came to me and were really curious about what they should believe about the moral status of animals, I would point them to contemporary debates. I would not suggest looking into the long history of thought discussed in Steiner's book, even though I find it very interesting.
This is because the moral status of non-humans is really not that complex an issue. It's an important one, and it's certainly difficult in a variety of senses, but really there are only a few relatively plausible views. And the arguments in general are relatively simple. Most people agree that animals matter morally. This must mean that we cannot cause them unnecessary pain or suffering. Unless you accept some bizarre metaphysical views then we also can't unnecessarily prematurely end their lives. These simple prescriptions rule out nearly all current use of animals. There are objections here and there to be answered, but all told, the answers are not very complex or surprising.
Why don't people just accept the arguments then? I'm not sure, you'll have to ask them. Part of the answer, I suspect, is that although accepting these arugments does not require a significant break from structure of ordinary moral beliefs, it is a break from an ordinary conception of the world and our place in it.
I like knowing what Descartes thought, and Plutarch, and St. Augustine. And knowing this history does give some important insight into my own thoughts, and the ongoing moral conversation, which is important in its own right. But really, if what you're looking for is a good answer to the question of the moral status of animals, all it takes is a little bit of reading, and a fair amount of reflection. The world would be a better place if people were more willing to do both.