I'm looking for recommendations for a good book on focus ( attention control ). I think this is an area of weakness for me and I want to read some on techniques for improving.

I've read a little of GTD and acknowledge it has some bearing on my ability to focus by getting things out of my head and into a 'system', however I am looking for something less holistic than that.

I find that even when I have a clearly defined list of things to do for the day I still tend to jump between tasks, on email, and start on other things not prioritized for that day.