Seems pretty simple. Don't imagine it provides recipes from all those books. That would be a bit too much to expect for $25 a year. They have, though indexed the recipes, and it looks like you can find which books have recipes fitting your keywords. You get that and a list of ingredients and go to the physical book for the full recipe.
Actually, it appears you can search all the indexed cookbooks, magazines, etc., and can see the ingredients list without signing up. Of course, you can't register a book as being on your "shelf" without signing on. And not a bad way to check out a prospective book purchase. You can go into the library and see the list of recipes in each book. Doesn't tell you how well it's written, but it tells you if it covers the sort of things you want. Interesting concept. Wish I'd thought of it.
Their list of most common specific books on users' shelves is kind of what you'd expect:
TOP 10 BOOKS
from our Members' Bookshelves
The Silver Palate Cookbook
Mastering the Art of French Cooking Vol I
Joy of Cooking
The Barefoot Contessa Cookbook
How to Cook Everything: Simple Recipes for Great Food
The New Basics Cookbook
Barefoot Contessa Back to Basics
Ad Hoc at Home
The Barefoot Contessa Family Style
Vegetarian Cooking for Everyone
"Kitchen duty is awarded only to those of manifest excellence..." - The Master, Dogen