Cookbook Library? Cookbook Store?
Certainly one candidate for the library would be the culinary collection at the Schlesinger library at Radcliffe (close to Harvard University). Barbara Haber is curator of this.
Nach Waxman's KITCHEN ARTS & LETTERS in NYC is one of my favorite cookbook stores in the US (where I'm assuming you are located). Powell's in Portland OR is another winner. Bonnie Slotnick's unique shop in Greenwich Village has out-of-print books and an interesting collection, though not on the scale of KA&L.
What a delightful gift you're giving your mother.
The Schlesinger library at Radcliffe is a great destination because you can combine the trip with a visit to New England Mobile Bookfair in Newton. The independently-owned bookstore discounts every book every day and has a huge section of remaindered cookbooks. If you want to get a sense of the size of the selection, visit Jessica's Biscuit, which is the online presence of the Bookfair's cookbook collection.