Translation:This book is as interesting as the last one.
I don't think you need "one" in the English translation.
"this book is as interesting as the last" should be accepted
I agree. Adding "one" in the English is fine but unnecessary. I love that Duolingo uses complex sentences but it seems difficult for them to capture all the reasonable variants.
Speaking of books, we don't say прошлая - rather, we'd say предыдущая (previous). The adjective прошлый collocates with век, год, месяц, неделя, раз, приезд - in other words, with nouns refering to time periods or events. "Прошлая книга" sounds awkward.
"такой же [adjective], как и" is a set expression. Although "и" can be omitted, it is much more common to use it in this expression. Without an adjective after "же" "и" becomes redundant. "такая же, как предыдущая" means "no different from the previous one".