Let's talk about quality aspects for book paper. We are a paper producer, so of course we may be partial, but this is what we have found out through experience. We have delivered book paper to the market since the early 2000's and produced paper for over 100 years!
Ultimately, the readers decide which quality aspects are most important. And the readers, buyers and decision-makers on the book market, can only choose from what's available on the market. This is why innovation, quality and sustainability ambitions of the paper producers and purchasing professionals are so important.
The most important thing regarding quality for book papers is that the paper is smooth and even, without disturbing flaws. This is the answer you will get, no matter who you ask - reader, bookseller, printer, head of production, publisher, author, everyone.
Surface smoothness varies between different products, but the paper's evenness is up to the thorough quality controls in the paper mill. An even paper sheet will run well in the printing machines, a basic requirement for achieving great print quality and minimizing costly downtime. Of course, responsive technical support together with predictable deliveries are also factors that can never be downplayed.
Readability: does the surface and the shade of the paper provide a reading comfort that goes hand in hand with the content and the design of the book? Technical data specifications will help you choose whiteness CIE (W) or brightness (ISO) for a comfortable contrast.
Thirdly, the tactile aspects of the paper: does it feel good to turn the pages, are they thick enough so that the content doesn't show through to the other side, and does the weight of the paper make the book feel heavy or light? Basis weight, opacity and bulk specifications together with paper samples will help you make the right choice.