For common, used, hardback books, i.e. recent popular fiction novels, I dribble white glue (Elmer's USA) down the fractured spine.
For my large paper-back Paul Simons 1985, I just use, and each page is now single. It was difficult from the beginning, terrible binding but great printing.
For personal publishing, I have used archival paper, Xerox printing, and a local library book bindery.
The 1880's leather bounds, sit in a bookcase behind glass...and disintegrate.
The mildew has yellowed my old textbooks.
The silver fish are making lace of note cards and files.
The 5.25 single-sided floppies of the DT are useless.