I think “Who Is Fourier? A Mathematical Adventure”, would be an example of the kind of book you are looking for. Its a top-down intro to Fourier transforms tha

I think “Who Is Fourier? A Mathematical Adventure”, would be an example of the kind of book you are looking for. Its a top-down intro to Fourier transforms that explains the derivation of the transform and why there are complex numbers involved, etc. Starts out very basic and top level, but covers all the important little details by the end.