Everything decided on screen is a proposal. Paper has the final say.

An uncoated stock absorbs ink and thickens every stroke slightly, which closes up counters and makes a delicate face look heavier than it did on screen. A coated stock holds the shape but throws light back at the reader. Neither is better; they simply ask for different type.
Thin paper saves money and weight, and shows the next page through. Where a book has heavy blocks of text or any solid areas, show-through turns from a technicality into a reading problem. Opacity is worth paying for before almost anything else.
A printed proof on the real stock answers questions a screen cannot: how the black actually reads, whether the margins feel right in the hand, whether the page turns cleanly. It is the cheapest insurance in the whole process and the step most often skipped under deadline.
By the time a run is on press, the decisions are made. The work is in the week before.
Even a book printed in one colour has a colour decision in it: how black the black is, and whether it reads warm or cold against the paper. Add a second colour and the question becomes how it survives on the chosen stock, which is rarely how it looked on the calibrated screen. Agreeing a tolerance with the printer in advance prevents the argument that otherwise happens on press day.