Marcella Hazan's books do what they say on the cover - they teach you how to make classic Italian recipes the traditional way. Marcella is not an innovator. Experimentation is not encouraged. Sample comment, "Fresh basil is a must. Some people may be tempted to supplant it with parsley, but is in no way an acceptable substitute." Well, that's me told.
However, if you're not from an Italian background, her books are extremely helpful in grounding you in traditional Italian techniques which you can then use as a basis to experiment with. For instance, it wasn't until reading her long rant about the quality of tomatoes these days that I tried making pasta sauce with good quality Italian tinned tomatoes rather than with fresh tomatoes and found that she's quite right, if you can't get good quality fresh Italian plum tomatoes, you are better off with good quality tinned Italian tomatoes. My pasta sauces have never been the same again. Of course, she'd still disapprove of me because I can't seem to find San Marzano tomatoes as she recommends but have to make do with Neapolitan. Her sauces - particularly the gorgonzola one and the tomato one with lots of butter and an onion which you take out and discard - are extremely good and, often, employ techniques that I would just never have come up with on my own.
In general, her recipes make me realise that I need to let the main ingredients shine when cooking Italian food and fight my instincts to overcomplicate by adding more and more spices/herbs/etc. Both books are dominated by recipes with meat but there's still plenty of good stuff that doesn't have meat and, to be honest, they're worth buying just for the sauce recipes.
Overall, very much recommended. But there's no way she'll ever persuade me away from my beloved garlic press. Sometimes, I even substitute parsley for basil.