You can see from the photo that this paper is very acidic wood pulp paper. The brownish tint and the rough ragged edges are a give-away. Acidic papers are very brittle and it makes restoration impossible and even preservation is very difficult. Acidic paper doesn’t want to bend. Any type of archival tape or tissue repairs will just create a new location for the paper to break along. Sadly, the book is doomed to disintegrate without any intervention. A de-acidification spray (which can be bought at a scrapbooking place) would cost quite a bit (maybe $18 per can) and it would take several cans to spray all the pages. The spray only stops the acidity from getting worse and cannot reverse it or make it stronger. But even gently reading the book is likely to cause further damage.
However, this is a special book for the owner and so I recommended using a bare minimum approach of Japanese tissue and paste (or document repair tape) to tack the pieces together.
If you have a book like this, I recommend buying a new copy that you can read while keeping this book carefully nearby.
Click Here for a Video on Page Repair Although it isn’t exactly perfect for this situation.
Basically, you would put the pieces into place with some waxed paper under that page. Then use tissue and paste to gently tack the loose pieces together. Don’t try to cover the area or go all along a break. Do only what is necessary so the pieces don’t fall out or get crumpled when you open or close the book. The hinging tissue would be a good weight of tissue to use. The Sekishu is too thick.