A core skill of interaction design is understanding how people think — specifically in relation to software systems but also on a general level. Most of this knowledge comes from observation & experience and the occasional psychology book in my IxD reading rotation.
I've recently come across a couple great sites that are useful resources for understanding people:
What Makes Them Click
The author has a Ph.D. in Psychology and offers great tips on how people think and relating that to software design.
You Are Not So Smart
I love the tagline : "A celebration of self-delusion". There are not many direct ties into software design, however the stories around each topics are fun and very useful in understanding how people tick.
This site starts from a UX focus and many of the myths center around how people really think. Each article contains lots of links to supplemental material.