A: Graded (sometimes called adapted) books are simplified classic books or written from scratch ones using simple English. Such books are graded by levels. The lower level - the simpler words and language constructions are used. Using graded books you can enjoy reading without much efforts and slowly increase your vocabulary. In every next book you will find some new words for you that will be used it this book several times in different sentences, so you can understand their meaning and remember them at once.
A: If you are only began to learn English you should start books from the first level too - Elementary. But if you has been learning English for a while, you can try Pre-intermediate and if it too easy for you - try Intermediate and so on.
A: Unabridged 'level' means that the book is not simplified in any way - it's original version from the author.
A: You can try of course, but if your own vocabulary will contain words from graded books only - unabridged ones will be very hard for you. There is a big distance between advanced graded and unabridged books. You should learn the full grammar course of English before try originals. Also after graded books, you could try magazines, journals and news at English language - they are much easier to understand.
A: Unfortunately, you can't. If you want to develop your English deep, you should not stuck at graded books because they can limit you. As soon as you can freely read advanced level books you should move on. But if English is just your hobby you can just enjoy reading graded books.
A: Now Read-online feature is available only for authenticated users. This means that you should Sing-up (register) and log-in at site. When you do so - Read Online button will be shown at every book page.
A: It's true, sorry for inconvenience, we are working on new level-calculation algorithm and soon levels of the all books will be re-calculated.