Goodreads.com is a website I use to track all the books I have read. With Goodreads, I can track my reading, keep a list of the books I want to read, and even add shelves to organize books based on genre. I can even rate and review books on the website so I can remember what I thought about the book when I first read it. Goodreads.com is also a social site, so you can make friends, follow other reader's reviews, follow authors, and just keep adding to your lists of books.
Goodreads.com also has a free app. While the app isn't my favorite place to write reviews (I personally prefer typing reviews on a full keyboard rather than a touch screen), it is a great place to keep track of the books you have read, look up books, and scan books to add to your shelves. For instance, let's say you are visiting a bookstore while on vacation and see a book you would like to borrow from the library, but are afraid you will forget it when you get back. You can open the Goodreads.com app and scan the book using the scan feature in the app, or just look it up using the search function. You can then just add it to your "To Be Read" shelf and look it up again when you get back to your library. The app can also help you remember if you have read a book or not. If you add all your read books to Goodreads.com you can search for the book, or search your "Read" bookshelf for the book to see if you rated or reviewed it.
I use Goodreads.com to keep track of all of my books. The app is a great resource you can take with you on the go.