There’s a lazy assumption built into the phrase ‘reading it again’ — that the book stays put and only your eyes move across it a second time. But the book you reread is not the book you first read, because the first reading changed you in ways that are now doing the reading. You know how the sentence ends before it ends. You notice the minor character because you already know what she becomes on page 300. None of that was available the first time, and its absence made the first reading a genuinely different event, not a rougher draft of the second.
I used to feel a faint guilt about rereading — time that could have gone to something new — until I noticed that the guilt assumed reading was mainly about acquiring content, book as data to be ingested once and filed. Under that model rereading is redundant. Under almost any other model of what reading is for, it isn’t.
The clearest case is a book you first read too young to understand it. I read a lot of things at fourteen that I now think I only recognized, without reading, in any real sense — see this note for one specific and slightly embarrassing example. Coming back to those books later isn’t nostalgia. It’s the first reading, just late.