Exactly as the title asks, I'm wondering what would be the best continuation to Kline's Calculus textbook. I honestly have found it to be absolutely excellent so far but where would I move to from there? Going to Spivak seems redundant but going to Apostol Vol. 2 seems like it'd be too much of a rigorous leap from the applied/physics-based Calculus that is Kline. Or is it? I'm not really sure... Please advise.