What I am aiming for, I remind myself, is to know where each book is, and to be able to put my hand straight onto the book I want without having to move anything else first. I didn't quite get to that point last year (still have 9 shelves with books stacked two deep!) But I think maybe I can achieve that now.
The other aim is to have relevant books, books which represent who I am today, not who I was x years ago. I don't know who said this, but I really like the concept. It helps me to be selective. And current.
(I did get down to one row deep on all but 4 shelves. Next year....)
Sorted bookshelves are now high on my list of life's simple pleasures!