I tried Khan Academy for a while, and while I love Khan dearly...
it just didn't fill in all the holes in my math education (but it did get me some solid practice with its almost endless practice problems, something that Ao PS doesn't have unless you use their Alcumus thing - but that doesn't cover every topic afaik).
After every few sections, there is an exercise set with no solutions for you to do.
To fully benefit from these problem sets, the authors recommend that you consult the solutions manual (if you order from their website it will come with the textbook) after giving the problems a good attempt or after you finished finding a solution.
While the Ao PS are phenomenal books and should be used instead of the terrible books used in middle and high schools today, I think you may want to look elsewhere if your primary interest for mathematics is to cover engineering mathematics.
The topics covered in these textbooks are mostly at a middle to high school level of mathematics.
To give you an idea of how they are written (at least from their algebra book), they are written in a tone of casualness to guide readers, typically younger students, into the concepts, many times having cute examples to go along with them (Captain Hook trying to find buried treasure comes to mind).
After each concept is presented, further concepts are explored through problems.
I've only worked through bits and pieces of the first few Ao PS books and I love them dearly.
The way they explain things really seems to click pretty well for me.