Is there a single source for the basics of cooking (sauces, herbs, oil usage, etc)?
I find that anything Cooks Illustrated puts out usually has a decent list of "whys" and have been garnering them up over the years.  Unfortunately this can leave you something of a specialist with deep knowledge on whatever their articles cover, but if you want to learn the why of something, they are a great source.

For baking in particular Ratio is an excellent book.