My favorite is still the first Jewish cookbook I ever bought: The Book of Jewish Food by Claudia Roden. Roden's family was from Egypt, and the book is nicely divided between Ashkenazi and Sephardic recipes, with solid, simple recipes for all. However, it does have the regular format divided up by food type, and a lot of simple traditional dishes, so it may not be what you are looking for.

A couple that I like with menus include Kosher by Design (which has a bunch of sequels, including a Passover one with various seder menus) and The Seasonal Jewish Kitchen. Both of these have modern recipes, with the Design books more fussy and Seasonal more rustic. Seasonal only has a few formal menus, but its organization by holiday/season makes it easy to pick a section and stick to a managable list of appropriate recipes without being forced to flip around too much.

https://www.amazon.com/dp/0394532589/?tag=mh0b-20&hvadid=7009917304&hvqmt=b&hvbmt=bb&hvdev=c&ref=pd_sl_1tj7s5fs3_b

The book above is by Claudia Roden. It's Amazing. It takes a much more anthropological view of Jewish food. It's equal parts cook book and cultural history. even from jews in china or india. amazing

https://www.amazon.com/Cooking-Jewish-Recipes-Rabinowitz-Family/dp/0761135812/ref=sr_1_1?s=books&ie=UTF8&qid=1494968407&sr=1-1&keywords=cooking+jewish

This one is collected from one family. it's great. much more practical as all these recipes are actually used by people in the family. Each one has a story about each dish.

Basically gonna echo most of the answers already posted, but just to pile on:

• 8" chef's knife. 10" is longer than may be comfortable and 12" is longer than necessary, but 7" may start to feel a little short if she's ever slicing large melon or squash. I'm a casual knife nerd and I have knives by Wusthof, Victorinox, Shun and Mac. My favorite.
  
  
• This Dutch oven. Enameled and cast iron just like the Le Creuset that a few other comments have mentioned, but much, much cheaper. I own two and they're both great. I also have the non-enameled version for baking bread, but I don't recommend it for general use unless you're a Boy Scout. Here's an entertaingly-written blog post comparing the Lodge vs. Le Creuset in a short rib cookoff.
  
  
• This cutting board and this cutting board conditioner. The importance of an easy and pleasant to use prep surface can't be overstated. I'm listing this third on purpose; this is one of the most important things your kitchen can have. A recipe that calls for a lot of chopping is no fun when you're fighting for counter space to do the chopping, or doing it on a shitty plastic board.
  
  
• A cheap scale and a cheap thermometer. Seriously, these are as important as the cutting board.
  
  
• Just gonna crib this one right off /u/Pobe420 and say cheapo 8–10" (I recommend 10–12" but that's my preference) nonstick skillet. One note I'd add is that pans with oven-safe handles are a bit more dual-purpose than pans with plastic or rubberized handles. You can't finish a pork chop in the oven in a skillet with a rubberized handle. But one could say you shouldn't be cooking a pork chop on a nonstick pan to begin with. The important thing is to keep this one cheap: you're going to be replacing it every couple of years, there's no getting around that. For my money $30 or less, and $30 is pretty expensive for these things.
  
  
  

• Cookbooks
  

  Nothing inspires cooking like a good cookbook collection. The great news about cookbooks is that they're often bought as gifts or souvenirs and they make their way onto the used market cheap and in great condition. Here are my suggestions for a great starter shelf:
  
  

1. The Food Lab by J. Kenji López-Alt. I kind of hate that this is my number one recommendation, but I don't know your wife and I do know J. Kenji López-Alt. This one is brand new so you're unlikely to find it used and cheap, but as a catch-all recommendation it has to take first place. Moving on to the cheap stuff:
   
   
2. Regional French Cooking by Paul Bocuse. This is possibly the friendliest authoritative book on French food out there, and a hell of a lot easier to just dive into than Julia Child (Julia is the expert, and her book is an encyclopedia). Bocuse is the undisputed king of nouvelle cuisine and people like Eric Ripert and Anthony Bourdain (so maybe a generation ahead of you and I) came from him. Paul Bocuse is French food as we know it, and yet this book—an approachable, coffee-table sized thing—still has a recipe for fucking mac and cheese. It's outstanding.
   
   
3. Theory & Practice / The New James Beard by James Beard. These will completely cover your entire library of American cooking. Nothing else needed until you get region-specific. When you do, go for something like this.
   
   
4. Essentials of Classic Italian Cooking by Marcella Hazan. When she died, the NYT ran a second obituary that was just her recipe for bolognese.
   
   
5. Christ, top five. Who gets 5th? I'm going with From Curries To Kebabs by Madhur Jaffrey. Don't get bamboozled into buying "Madhur Jaffrey's Curry Bible" which is the same book, repackaged and priced higher. You want the one with the hot pink dust jacket, it's unmistakeable. This is one of those end-all books that you could cook out of for the rest of your life. It covers almost every diet and almost every country that Beard and Bocuse don't.
   
   
6. Honorable mentions: Here come the downvotes. Pok Pok by Andy Ricker. If you're American and you want to cook Thai, this is the one. Ten Speed Press can go home now. The Book of Jewish Food by Claudia Rosen (so close to making the list). I shouldn't need to say much about this; it's the book of diasporic Jewish food, which means it covers a lot of time and almost every possible country. It's a no-brainer. Thai Food by David Thompson (a perfect oral history of Thai food for English speakers, only it doesn't include Pok Pok's precise measurements, which in practice I've found important). Flour Water Salt Yeast by Ken Forkish. Not for someone who just wants to become a baker, this book is for someone who wants to make Ken Forkish's bread. And for a casual bread baker I can't imagine a better introduction. Pleasures of the Vietnamese Table by Mai Pham. Andrea Nguyen is out there and Andrea Nguyen is awesome, but I really like Mai Pham's book. It's accessible, reliable and regional. You don't get the dissertation-level breakdown on the origins of chicken pho that you get from Andrea, but the recipe's there, among many others, and it's fucking outstanding. Veganomicon by Isa Chandra Moskowitz and Terry Hope Romero. This vegan cookbook is dope as hell and will really expand your imagination when it comes to vegetables. This could actually have been number five.

The classic is probably The Book of Jewish Food. Almost every Jewish kitchen I've come across has a copy, since it has recipes for all the well-known dishes (challah, gefilte fish, honey cake, bagels, etc), as well as hundreds more that aren't so common. If you only want one book, this will provide almost everything you need. If you want more of a library I agree that the Kosher by Design books are a good investment.