You build a much better understanding from problem sets than reading and taking notes. A method that I've found to be efficient is to answer as many problems as I can, and then in my own words write out my notes on the subject. Focus on tricky or difficult problems if you can, rather than doing...
I'm not entirely sure it would, to be honest. Khan academy doesn't have enough practice questions, and the ones it does have aren't very challenging.
Plus, I don't believe video lectures replace learning straight out of a textbook at all. You're not always going to have Khan to hold your hand...
Boas is a math methods for physics and engineering. It has less emphasis on theory, and goes over different subjects such as LA, DE's, vector calc, basically everything an undergrad physics major will need. If you're a physics major It will benefit you tremendously to work through it.
I would avoid the "For Dummies" series of books, there are much better options out there (though admittedly textbooks will cost you a lot more).
I used Precalculus: A Right Triangle Approach by Lial/Hornsby, and it had great coverage of material and hundreds of exercises for every section. It...
This is how it usually turns out when you're in school, and it's I find it difficult because I absolutely hate reading books out of order. I always get the feeling by skipping a chapter or two I'm missing out on extremely important knowledge. It's like book OCD.