If it’s only against yourself, it would be nice if the little Rank icon next to the challenge showed your percentage as well. Also, there is no need for a leaderboard - does it show how many books someone has read? No, seems just to be how far people are through their book!
And as someone else has pointed out, it’s kind of pointless when there is such a disparity in the size of ‘books’. Looks like someone just shoe-horned an idea into an existing structure.
This is one ‘challenge’ I think I’ll leave.