To an extent, all books have impact. The first impact is on you as the reader making you decide whether you loved the book or not.
But when I talk about books with impact, I mean those stories which stay with you long after you’ve read them initially and which you will happily re-read at any time.
For me, this includes The Lord of the Rings, Pride and Prejudice , most of the back catalogues of Agatha Christie, P.G. Wodehouse, Terry Pratchett, and the classic fairytales.
What pleases me now is I’m increasingly adding non-fiction to that mix. It makes for an interesting “reading diet” and I would include most of Ben Macintyre’s books in the “will happily re-read” category.
I don’t mind the book format I use either. I mainly read paperbacks and ebooks with hardbacks every so often coming into the mix (but I have to be very sure of the author for those!).
The impact I look for in fiction is entertainment.
For non-fiction it is in learning something new to me in a way that entertained me (and I welcome the development of creative non-fiction here because this is one of its strengths).