Even though I treat my books very carefully, the spines broke on a few of them. I don't like this as they then always open in the same place.
About 8 years ago I developed a technique to repair those books as invisibly as I could using shims of card and a certain brand of white shoe "glue". It worked very well and all the repairs held.
Hardback books are usually easy to repair. Softbacks are usually more difficult if you want the repair to be unobtrusive.
In the list of books numbers 50, 107, 124, 125, have been repaired.