With all the information available these days about Spirit, it's unusual to find a new source. But Rocket Man Blog is just that. Mark Oakley works in a test lab somewhere in the space industry:

My lab was involved in the testing to determine what the most likely cause of the Mars Polar Lander failure was.

His inside comments on the Mars expedition are fascinating. And he has a great play-by-play commentary of this amazing animation of the mission: How Spirit Got to Mars.