Harold McGee has two main book. On Food and Cooking is his more famous and 'hard core' book and definitely not something I would recommend to beginner cooks. The ratio of scientific background to useful advice is much more geared towards someone who really wants to deeply understand cooking. It also spends a lot of time talking about the history and anthropology of cooking, which while interesting, isn't useful if you just want to learn how to make a tastier omelet.

The Key To Good Cooking on the other hand is a much better book for home cooks and focuses entirely on practical ingredients and techniques and is organised in a way that makes it much easier to find exactly the advice you need as you need it.