These are the books that I go back to over and over again. Hardcover only.
Salt Fat Acid Heat
This book teaches you how to cook. This kicked off my love of cooking. Made everything so much more fun. It's not a book of recipes, it's actually about how cooking works and what is happening to the food. After reading this, making a dish felt less like following directions and more like improvising with a set of guidelines. Much more fun, much more creative. Fast and loose.
Essentials of Classic Italian Cooking
This book is just the best. It has everything you need to make good Italian. Need a pesto recipe? Boom. Risotto? Yes. Fun fact, this is also the base for my bolognese recipe, which those who know me know is the best thing I can make.
Back Pocket Pasta
I've made ~10 pastas from this. All good and all easy. Also, this is a goto gift for people that like Italian food.