I've read two of those books by Isham: The ones about quantum mechanics and differential geometry.
About the QM book: It's a very good presentation of QM that's tells you more than most books about what the theory actually says, but very little about how to calculate stuff. So it's about understanding the theory, not about how to use it. I think it can be studied by people who haven't studied QM before, but I think it's intended mainly for people who have taken one of the standard introductory courses already, e.g. a course based on Griffiths. I highly recommend it, regardless of what you have studied before.
About the differential geometry book: This is very nice introduction to some aspects of differential geometry. I really liked how he covered tangent spaces for example. It also has a very nice introduction to Lie groups, Lie group actions, and fiber bundles. The level of mathematical rigor isn't as high as in a math book. He will often not bother to prove that a function has all the properties we need it to have. If we e.g. need a smooth bijection, he'll just prove that it's a bijection.
The goal isn't to teach the differential geometry that you need for general relativity. It's to teach the differential geometry you need to understand Yang-Mills theory. I think he has successfully accomplished what must have been one of his goals: to make his book the best place to start a journey towards Yang-Mills. But I think it ends at a weird place. I would have wanted to see two more chapters or so that cover differential forms, integration on manifolds, and field theory Lagrangians, so that the reader actually gets to the point where he understands the Yang-Mills Lagrangian.
The last chapter, about connections and curvature in fiber bundles, isn't as well written as the earlier chapters. I got the impression that he was in a hurry to finish the book. It takes a lot more work to understand what he's saying there, but it's not just because of the writing. It's the most difficult subject in the book, and it's probably still easier to read about it in this chapter than in another book. All things considered, it's a very good book, and I highly recommend it to everyone who wants an introduction to the things I mentioned above.
However, I think a person who wants to learn the differential geometry needed for general relativity should study Lee instead. Read his "Introduction to smooth manifolds" to learn about manifolds, tangent spaces, tensors, differential forms and integration on manifolds (and also Lie groups if you're interested in that), and read his "Riemannian manifolds: an introduction to curvature" to learn about connections, parallel transport, geodesics and curvature.
For group theory, consider the book by Brian Hall. It focuses on matrix Lie groups, the corresponding Lie algebras, and representation theory. By focusing on matrix Lie groups, he's able to avoid differential geometry altogether.
For linear algebra, the most popular books around here are the ones by Axler and Hoffman & Kunze. I have only read the former, and I think it's really good, and covers precisely the topics I'm interested in (the stuff you need for quantum mechanics). I've been told that Hoffman & Kunze covers a wider range of topics and that their explanations are more detailed. I don't like Anton, because he doesn't define linear operators until around page 300. I find that completely ridiculous.