My father-in-law (RIP) used to be a paper engineer and he had a huge collection of paper at home. He used to make his own paper for fun and I had the pleasure to assist him on a few batches. Interesting, if time-consuming hobby.

Anyhow, comparing different paper types you can see, feel, and smell the differences in quality. It starts with good raw materials, recycled low-quality paper will never make high-quality paper. That's because for high-quality paper you want long cellulose fiber in the paper. The longer the fiber the hardier the paper. The less acidic the paper, the better. You add chalk to make paper less acidic. The best paper has very long cellulose fiber and is virtually acid-free. If your paper turns yellow and "brittle" over time it is because of acid.

Now, the paper-surface is treated to create different effects (e.g. glossy, water-resistance). IIRC, that is called "coating". Coats may introduce acid again. Note that untreated paper is rather smooth and yellow-white-ish. To get this recycled, natural, rough look that is en vogue at the moment, you actually have to treat the paper to look like this. From an environmental point of view you would be better of with a smooth white paper. For longevity you want to coat your paper with an acid-free solution.

It is a little bit ironic, but engineering improvements in paper manufacturing allowed us to produce paper with lesser and less raw materials and worse pH values (e.g. industrial mills need far less water for a ton of paper than a century ago). This cheaper paper replaced the cheap paper from before, therefore degrading the paper quality, making modern cheap books less hardy than old cheap books. Cheap paper from 1930 will last a century or longer, cheap paper from 1970 will last maybe 50-60 years under normal storage conditions, ie. in an open bookshelf.

Now, that's only the paper. The printing ink and the binding also play a role in longevity. First, ink adds acid to the process, which is always bad, but depending on the ink type (dye or pigment, fountain pen enthusiasts will know this) the ink itself will fade faster or slower. Ancient inks are all of the pigment variety (or maybe at least those we know of). They have very long staying power. Modern inks are mostly dye and as a rule of thumb add more acid and fade faster. Bindings have little impact on readability, but are of course vital to the survival of the book as a book. A glue binding will degas over time. It becomes hard and breaks in a few decades. When single pages break out of the book you see the cheapest of the cheap bindings. Exposure to heat will accelerate this process, so if you like your glued books, keep them out of the sun.

High-quality books are saddle-stitched and work differently. First of, you do not stitch individual pages but fascicles, a bundle of paper, each paper holding four pages, which are then again stitched together in bigger bundles and finally into the book cover. This requires some forethought in the layout of the book and is very, very expensive. I own an archival hardcover print of _Also sprach Zarathustra_ from 2002, which was sold for about 300 EUR at the time. It was gifted to me for some accomplishment then. That's a book truly in its own league. I own a few other archival hardcover prints, but none this good. But I digress. I wanted to say that with good paper, coating, ink, and proper storage those books have virtually no end of life.

Sorry for the long post, brought up a lot of memories.