"This is the most interesting book that I have ever read"???

I understand the sentence "This is the most interesting book that I have ever read." as meaning that it is the most interesting of all the the books that the person has ever read in his lifetime.

But why is this so? Does the relative pronoun "that" include the meaning of "of..." or "among...," or is there some ellipsis?

"This is the book that I read yesterday." means that it is the very book that I read yesterday and nothing else. No confusion there. However, in the case of "This is the most interesting book that I have ever read," there are more than one book that are stipulated in the sentence, namely the one that I have just read and found the most interesting and all the other books that I have ever read so far, all of which are less interesting than the aforementioned one.

Likewise, I don't see why the sentence "This is as interesting a book as ever was," which may be transliterated "This is as interesting a book as any that ever existed," is understood as meaning what it's supposed to mean. (I know what it means, but don't see why it means so in terms of grammar.)