Note: in calculating voltage from a given electric field, I use very different approach from the book which is easier and does not require understanding of path integral. The technique is demonstrated here in the review session and also in my lecture notes. I
1. integrate a component of electric field (most of the time this turns out to be integral with respect to r, because of general spherical and cylindrical symmetry of "askable questions'). This is indefinite integral which ends with something + constant.
2. Find constant by matching to the boundary conditions such as V=0 at infinity or V=certain value at the boundary of a sphere for example.