Good question Jeannie,
It's 21 walks the lowest being 715m (Brandreth).
The book's by John Drews, and the hardest looks like the Sca Fell day, 8 peaks, 4888ft of climb, 7.5 hours (ish), although the Helvellyn, Great Gable and Bow Fells days look tough too. I think high summer days (week days only) would be required for them, as they are also quite busy routes by the look of them.
The idea of the book is to "DO" the 100 highest peaks and tops, in circular routes, with minimum backtracking and minimum overlapping of routes.
Can't wait to get cracking!