With the guide book you could walk it, record it with a GPS then upload it as the "name " trail as a public trace ( I am not a legal expert but copyright holders of the name may not allow you to use THE name but you could use something descriptive such as A to B). Then You or another mapper could create a route relation for it. If you look at waymarkedtrails you can see whats been mapped already. The routes can be looked at and a GPX can be downloaded. I helped with this one. https://help.openstreetmap.org/questions/34347/i-want-to-add-a-new-long-distance-hiking-route-to-the-map edit after your comment:- You could contact the copyright holder and ask them if they would allow use of the name. You could suggest that the tagging could mention the book and they may get book buyers from walkers finding the route on trail map sites who like to read a guide book. A charity walk in the UK is the Macmillan Way a lot of people walk it, A lot them sponsored hikers that collect donations for that cancer care charity by walking it. So Macmillan Nurses have just got a free plug for their excellent work .. because it's on a map.

With the guide book you could walk it, record it with a GPS then upload it as the "name " trail as a public trace ( I am not a legal expert but copyright holders of the name may not allow you to use THE name but you could use something descriptive such as A to B). Then You or another mapper could create a route relation for it. If you look at waymarkedtrails you can see whats been mapped already. The routes can be looked at and a GPX can be downloaded. I helped with this one. https://help.openstreetmap.org/questions/34347/i-want-to-add-a-new-long-distance-hiking-route-to-the-maphttps://help.openstreetmap.org/questions/34347/i-want-to-add-a-new-long-distance-hiking-route-to-the-map edit after your comment:- You could contact the copyright holder and ask them if they would allow use of the name. You could suggest that the tagging could mention the book and they may get book buyers from walkers finding the route on trail map sites who like to read a guide book.

With the guide book you could walk it, record it with a GPS then upload it as the "name " trail as a public trace ( I am not a legal expert but copyright holders of the name may not allow you to use THE name but you could use something descriptive such as A to B). Then You or another mapper could create a route relation for it. If you look at waymarkedtrails you can see whats been mapped already. The routes can be looked at and a GPX can be downloaded. I helped with this one. https://help.openstreetmap.org/questions/34347/i-want-to-add-a-new-long-distance-hiking-route-to-the-map