Books come in many sizes-- there's the mass market paperbook that you might "curl up with" or read on the train. Those are approximately the size of a large phone or kindle. Then there are large books-- atlases, art books, books such as
Audubon's Birds of America and what are popularly known as "coffee table books". The iMac 5K does a good job of displaying those sorts of works-- if you enjoy looking at
illuminated manuscripts, the printed page looks amazing.
But most digital sources don't have the ultra high resolution to look their best on the imac screen. You can sometimes turn off resolution scaling to force remote image servers to give you the best copy-- I'v had to do this with the new yorker archive, but in many cases, the full 5K resolution exceeds what's been digitized.
You can also read text on the imac screen, and it will look amazing. If you have to be sitting down at a desk, there's nothing wrong with reading text on a imac screen. But if you don't have to be sitting down at a desk, an ipad or ereader are sometimes more comfortable to use for pure text.