In the Hazan case, Essentials of Classic Italian Cooking does combine Classic Italian Cooking and More Classic Italian. but she edited a number of the recipes, changing ingredient ratios and such so they're not exactly the same as in the earlier books.
I have a bunch of cookbooks, both in paper and ebook (I don't love using them in ebook form, but it's hard to resist snapping up a book I'm interested in when it's selling for only a few bucks), and I use Eat Your Books to catalog them. It's a pay service, but it is super useful for keeping track of and making the most of cookbooks because you can search your bookshelf for recipes using a variety of criteria, and organize them in various ways. Even without a membership, you can use it to browse through or search the recipe titles and main ingredients of indexed books and blogs, access recipes online, and read people's recipe notes. They have a ton of giveaways, too. Here is my personal bookshelf.