I would strongly advise against gluing it yourself if you have any intent to try to use it for anything other than as a personal souvenir. And if you were to try to use the beaten up passport for a new/replacement passport, it's better to not do any glue fix yourself.

The more obvious it looks like its condition is that of an extreme but naturally worn and torn passport book -- without any attempted alteration/fix to conceal even damage -- the better it is. Signs of attempted alteration for even fix-purposes is more of a warning sign than a badly aged passport desperately in need of a replacement.