The Organizing Sourcebook by Waddill
Because while it's great to have a ton of ideas on how to deal with say, a pile of mail, it'll do you no good unless you're solving your actual problem.
TOS walks you through simple basic guidelines for looking at the things you need to get done in your life and setting up systems for making them work.
For instance, she gives an example of three different messy dining room tables. Three different solutions:
In one, the dining room was reconfigured as an office
In another, better lighting was put in the room that was supposed to be the office,
in the third, they used a rolling filing cart so that work could be done out where mom could be with the kids, but then all the office stuff tucked away quickly when the bills were paid.
It will greatly improve the usefulness of any of the other books.