Even though I allowed space behind the book shelves for air to circulate, the books facing an outside wall are browning on the inside cover and discoloring splotches are appearing on the edge of the paper, side and top, (I suspect my house isn't insulated), while books on shelves facing an inside wall don't appear to be aging at all, even new paperbacks purchased in the sixties. Except, for some reason, some front covers stick to the back covers of the books next to them and parts of a cover will rip off when separated. My copy of TO YOUR SCATTERED BODIES GO has a bunch of measle-like white spots all over it from sticking/ripping. I may have my shelves too tightly packed.
That, and a slight problem with humidity -- perhaps not enough for you to notice it, but enough for the moisture to react with some of the chemicals use in production with certain books. A drier environment tends to eliminate that problem... unless you have your shelves
really packed, enough to make it difficult to remove a book....
Clansman: I've found that, with what you were saying, the worst period was from around 1895 to the mid-1930s (or, with some companies, the mid-1940s). For example: a first edition of the American release of H. R. Wakefield's
They Return at Evening which is still in very good shape is quite hard to find; the paper is so fragile that it flakes, no matter what you do; while my copy of Edward Lucas White's
Lukundoo and Other Stories from the previous year is in almost pristine shape (save for missing a dustjacket); ditto for my copy of
The Song of the Sirens, while my copy of Holmes'
Elsie Venner is also quite fragile. But books from 1890 are often in much, much better shape, showing scarcely any browning at all; the paper is firm (if not always limber -- though sometimes it is); and the sewing on the bindings is still strong, whereas with books from that period it is often so thin and fragile that it can snap at a moment's notice.
Paperbacks, on the other hand, are quite another matter, as the paper used in them was of close to (or exactly) the same grade used in the pulp magazines; hence is falling apart by this point (as with my copy of a 1940s edition of
Creep, Shadow! by A. Merritt). At the same time, as I noted earlier, certain book clubs at any rate, were using a higher-grade paper as early as the 1940s at least, and some of those I have are still in remarkably good condition....
