I think it would be helpful to learn the language of manifolds and differential forms for differential geometry and relativity. Spivak treats those and thus would be helpful, since apparently apostol does not. I myself read spivak and benefited greatly. Since his treatment is very terse, it might be easier to learn manifolds and forms from one of the other more accessible sources mentioned here, perhaps Munkres, Hubbard, or Shifrin, but I myself am not as familiar with those. People also like books by Lee.
Of course many elementary books on diffrential geometry begin with intro to manifolds and forms. There are also nice books just on differential forms, e.g. one by Henri Cartan, one by Harley Flanders, and one by David Bachman. Since they are kind of unintuitive and abstract as usually presented, it helped me a lot to just see how to calculate with them in a little article iin an AMS monograph by Flanders. I can't find that reference. The point is that just learning how easy it was to add and multiply them was very reassuring considering how abstract their definition was.
You don't really need to worry about all this information at this point however, since it will may well take you quite a long time to master Apostol. I.e. it is more useful to actually read one good book, than to make a long list of books to potentially read. I have not yet read every word of spivak's little book on manifolds, nor much less even volume 1 of his enormous differential geometry book, and I started them in the 1960's.
Still, working through chapters 1-4, especially 4, of spivak's calculus on manifolds, taught me a lot. Unfortunately his notation for the proof of stokes' theorem on about page 103 of calc on manifolds made my eyes glaze over. I never understood how easy it is until I read the little discussion in Lang's Analysis I, on p.442, Stokes theorem for simplices, in the case of a rectangle. Probably you would get this from Apostol, which I had not read then. I.e. stokes theorem is trivial, you just use fubini's theorem to reduce it to a repeated integral in fewer variables, and then it reduces to the one variable fundamental theorem of calculus. But it is hard to understand abstract brief versions of something unless you have first studied the basic classical versions in detail in low dimensions. After that it is nice to see how the details can be made elegant and the concepts unified.
good luck!