I’ve recently discovered a bunch of math / physics Dover books via Google Books of all things. You can access previews of them based on the link below and also buy them. I like this because I have a tendency to splurge for Dover books only to find my math skills are too weak to make progress. I don’t mind, since I like giving money to Dover, but my bookshelves are too full and this seems like a good compromise. Here’s just some I found good:

1. Mathematics of Classical and Quantum Physics
2. Mathematics for Physicists
3. Mathematical physics
4. Classical mechanics
5. Foundations and Fundamental Concepts of Mathematics

Also I found these good links, not Dover or Google Books necessarily, but worth checking out:

1. Calculus and beginner calculus
2. History of physics
3. Principle of least action
4. Lagrangian physics and why it is useful.