The "Donnie Darko" website includes some extra tidbits. Allow pop-ups for everything to work properly.

Level 1 has pages from the Roberta Sparrow's The Philosophy of Time Travel.

http://www.donniedarkofilm.com/

The Donnie Darko Book has the film script, pages from Roberta Sparrow's The Philosophy of Time Travel., film stills and photos of "Donnie Darko" artwork (mentioned in featurette on the DVD).

For more "Donnie Darko" trivia:

http://en.wikipedia.org/wiki/Donnie_darko

The "Director's Cut" explains the time travels ideas a bit better.

Here's the official website for the film. The address changed recently:

http://www.donniedarkofilm.com/

In the area that concerns Roberta Sparrow, there are pages from the book on the philosophy of time travel.