Few mathematicians go through all the topics of advanced calculus and master the proofs of all the technicalities. They do enough of such work so that if the need arises they can go back to the material to understand any proofs they need. Most mathematicians deal with the proofs in advanced calculus by regarding them as special cases of results in a more advanced field called "functional analysis".
I have Widder's "Advanced Calculus" 2nd Edition. In my opinion, trying to go through that book page-by-page would not be the most effective use of your time. I doubt the "average bright person" would make much progress in a single summer. Widder is rigorous relative to the standards of a typical introductory calculus text, but his subject matter is not very general or abstract. I would recommend Widder's book to engineers. I'd recommend that a person pursing mathematics spend their time on a topic more general than "Advanced Calculus". (So I probably wouldn't recommend Edward's book either, but I haven't seen it.) If your goal is to pursue math, try something in the field of "Calculus on Manifolds" or "Introductory Real Analysis".
Advanced Calculus by Widder
Table of Contents
1. Partial Differentiation
(includes Taylor's Theorem in sevveral variables and Jacobians)
2. Vectors
(includes gradient, inner product, outer product)
3. Differential Geometry
(arc length, osculating plane, curvature and torsion, Frenet-Serret formulas, differential forms, Mercator maps)
4. Applications of Partial Differentiation
(includes Lagrange multipliers)
5. Stieljes Integral
(includes Heine-Borel theorem, uniform continuity)
6. Multiple Integrals
(includes Duhamel's theorem, center of gravity, moments of inertia)
7. Line and Surface Integrals
(includes Green's theorem, exact differentials, Gauss's theorem, Stoke's theorem)
8. Limits and Indeterminate Forms
(includes l'Hospital's rule, limits points of a sequence, Cauchy's criterion)
9. Infinite Series
(includes the usual tests for convergence, differentiation of series, Cesaro summability, Cauchy's inequality, uniform convergence)
10. Convergence of Improper Integrals
(includes the usual tests, uniform convergence, divergent integrals, Schwarz inequality, Minkowski inequality)
11. The Gamma Function. Evaluation Of Definite Integrals
(includes Stirling's formula, the Beta function, Euler's constant)
12. Fourier Series
13. The Lapace Transform
14. Applications of the Laplace Transform
If you are tutoring students who are taking Calculus I and II, most of those topics won't be relevant.