I actually read the book, which covers all levels. Most people who get into GTD have problems with the lower levels, thats why so much is written on them. But David Allen does alot to cover the highler spheres too, both in his lectures and his writing.

That's the remaining 10%. You know, the part which isn't covered in 'teach yourself GTD in one hour' audiobooks.

But seriously, there's much stuff about higher levels of planning in GTD. See 'someday maybe' lists, monthly review, putting analysis tasks on monthly lists, analysing farther horizons periodically, etc.