I get asked about my opinion of the NPP at events and on Q & A episodes of the Naked Bible Podcast. I always recommend Kent Yinger’s book, The New Perspective on Paul: An Introduction. It’s short and clear.
I’m happy to also recommend this brief set of comments from Michael Bird on the issue. It won’t educate you like Yinger’s book, but Bird captures my own thoughts. In a sentence: “The NPP is correct in what it affirms, but often wrong in what it denies.” That’s well said. Read what he means at the link (and get a lot more with Yinger’s book — the same spirit there).