courant does not go into things quite as precisely as apostol. courant is deep, but there is a little more taken for granted. i think courant will assume basic facts from trig as needed, whereas apostol will lay out exactly which proeprties of sina nd cos are going to be used.
they are basically the same properties, but apostol has a fanatical commitment to really spelling out every fact and assumption with complete precision and care.
you can get the idea by going to the library and perusing these two boks, and then comparing with a book like stewart or anton, something along those lines.
apostol and courant are answering questions that do not even arise in books like stewart et al.