Cartwright's analysis is not easy to follow, but it helps to appreciate that the title is intentionally a pun. She is addressing the question of where the laws of physics stand, epistemologically (how they "lie") as well as how they fail to capture the full complexity of the world they are intended to describe (and thus how they are not true in the sense of giving a complete, God's-eye view of how things work). I think the critical idea is that the laws of physics are concepts that abstract away from a great many messy details that characterize things like interactions among bodies in the real universe. Each law thus comes with a ceteris paribus (all else being equal) clause attached. So, for example, the ideal gas law tells us how pressure, volume, and temperature are related, but it is reliable only for closed systems. When she says that such laws are not very useful she means something quite specific, namely, that such a law is, by itself, almost useless for understanding p-t-v relationships in open systems, like the Earth's atmosphere, where all else is NOT equal. Such laws are extremely useful as foundational concepts in our abstract understanding of how the universe works, but it can take years, or decades, or even centuries after the discovery of a law for engineers and technologists to figure out how to cash out all of the "all else being equal" clauses in the real situations where the laws operate. For example, the central laws governing fusion in plasma are pretty well understood, but turning that understanding into an operating fusion generator is proving extremely difficult. I do agree that there is some slippage in how the word "true" is being used in many of her discussions, with insufficient discussion of some important nuances, but far from rejecting realism, I think she is giving us a very important and powerful new set of conceptual tools for understanding what realism actually requires, epistemologically speaking. In summary, I think this is an extremely important argument in the philosophy of science, one that someone who wants to defend realism and the notion of objectivity has to understand and appreciate. It's tough going, but worth the effort.