I think cookbooks have a few advantages:
1) Someone had to come up with a lot of money up-front to have the books printed. Nobody is going to spend that kind of money without making sure the author is an expert & the recipe are tested. It ensures a base level of quality. OTOH any idiot can post a recipe online, so on average cookbook recipes are much higher quality than online.
2) For beginners. Cookbooks provide the information needed without being an info overload like the internet can be.
And personally, I prefer having a cookbook in the kitchen rather than an electronic device. I'm less worried about getting a spill on a book than on an expensive device. Plus books don't need charging, are usually larger and easier to read than mobile device screens, etc.
La Methode and La Technique - Jacques Pepin (or the later compilation "Complete Techniques")
Mastering the Art of French Cooking - Julia Child
Joy of Cooking (especially the 1960-1970s editions)
Good Housekeeping Illustrated Cookbook
The Professional Chef (current edition is expensive, previous editions very economical)
Charcuterie - Ruhlman (sausages, cured meats)
Bread Baker's Apprentice
Pok Pok - Andy Ricker (Thai)
Thai Street Food - David Thompson (Thai)
Land of Plenty - Fuchsia Dunlop (Sichuan Chinese)
Mastering the Art of Japanese Home cooking - Moriomoto
Iron Chef Chen's Knockout Chinese - Chen Kenichi
Mrs. Beeton's - British
Heritage - Sean Brock (US Southern)
Into the Vietnamese Kitchen - Andrea Nguyen
Anything by Steven Raichlen (Grilling and BBQ)
Franklin Barbecue - Aaron Franklin (BBQ, especially brisket)
Various books by Gordon Ramsay (Excellent if the topic is British/French; other cuisines meh)
On Food and Cooking - Harold McGee (science, background, theory)