+1 for Apostol V1! I also read it in high school, ended up testing out of Calc 1 in Uni, having learned plenty from reading that on my own. As you say, it really is so conversational and easy to follow. Starting with integration works really well, you build lots of sums and then use a supremum/infimum as opposed to a limit. I think the mental imagery for that is a lot more manageable than limits, especially if its your first time watching infinities disappear.