there are three concepts in one variable calculus, continuity, differentiability, and integrability. Then there are several basic theorems: intermediate value theorem (basic property of continuity), existence of maxima and minima of continuous functions on closed bounded intervals, and mean value theorem. the key theorem relating these concepts is the fundamental theorem of calculus, showing how to compute integrals by anti differentiation. There is also the inverse function theorem, largely a corollary of the intermediate value theorem.
In several variable calculus the same three concepts occur. Basic theorems are the inverse function theorem, the fubini theorem (reducing an integral of several variables down to a sequence of one variable integrals), and the stokes theorem, (generalizing the fundamental theorem of calculus).
Other common topics include summability of infinite series, and solvability of simple differential equations, especially linear ones with constant coefficients. Then there are applications of these ideas to computing volumes, and physical concepts like work. One good book is an old version of the book of George B. Thomas, formerly available used for a couple of dollars. Omiword, those old books are now hundreds of dollars. It seems to be understood that the newer books with names like Finney, Haas, Weir, Giordano, on them are greatly inferior. But maybe the alternate 9th edition with just Finney on it is ok.
https://www.abebooks.com/servlet/Bo...centlyadded=all&cm_sp=snippet-_-srp1-_-title3
But I suggest going to a library for an older edition from the 50's or 60's.