Presumably the one by Charles Duhigg [1]. It was very popular last year.

[1]: http://www.amazon.co.uk/The-Power-Habit-What-Change/dp/18479...

This is the least data-backed, and most drivel-filled article I have read on hacker news. Seriously?

Anyway I strongly recommend "The power of habit: Why we do what we do, and how to change" which is a scientific investigation into, well, the science of habit, but more importantly the information contained in those pages is highly usable.

http://www.amazon.co.uk/Power-Habit-Why-What-Change/dp/18479...

Not to mention the "Don't break the chain" principle, which inadvertently makes you apply the most fundamental principle of good habit building:

https://news.ycombinator.com/item?id=36497