how to be less rigid about adhering exactly to recipes

cook more by look and feel than by very precise measurements

Both volume and weight are inaccurate. And if you can find ways to measure less, your cooking will go faster. So learn to cook by look and feel.

learn to cook based on what's in your fridge, with loose "frameworks" instead of strict recipes

This is the entire point behind the CookWell.com project.

ingredient substitutions

The 700-page (wow!) book "The Food Substitutions Bible: 8,000 Substitutions for Ingredients, Equipment and Techniques".

TODO: Add more here.

fixing too much salt / acid / etc

The 200-page book "How to Repair Food" by Tanya Zeryck, John Bear, and Marina Bear