It's really hard to find good writing on metaethics. My recommendation would be to read the chapter on ethical reductionism from Miller's introduction to contemporary metaethics, my own unfinished sequence on metaethics, and Eliezer's new sequence (most of it's not metaethics, but it's required reading for understanding the explanation of his 2nd attempt to explain metaethics, which is more precise than his first attempt in the earlier Sequences).