If I remember rightly from the Digicube website, this was a four book series on Final Fantasy: The Spirits Within, each covering a different aspect of the film.  So far I managed to get two.  Unfortunately, Digicube recently went under, meaning tracking down the others is going to be a task and a half.  Er... and for those of you who don't know, Digicube was a sub-company of Squaresoft that handled merchandise, strategy guides, and some games.

The two I have are Final Fantasy: The Limited Collection and Final Fantasy: The Future Science.

The Future Science, from what I can tell, details the science of TSW and compares it to the science of now (or a couple of years ago).  For example, it includes things on the UN and the League of Nations (I think) in comparason to the movie's Council; biology to bioetherics; military technology to military technology; holograms; and so forth.
 

The Limited Collection, my word.  This book... er... well, wait.  It's more of a case.  Okay, so... this case opens, and you can get the CD case and book from within.  The CD contains CD-ROM information - well over a hundred screen captures for computer wallpaper, a screensaver, if I remember right... lotsa stuff, I'll settle for.  The book... The book opens two ways.  The 2001 side, where there's story information and such from our point of view.  It has the timeline, and cast and such.  And then there's the 2065 side, which reads like a databank from inside the movie.  It's literally called something like, "The internet of 2065."  It has person, place, thing information... on more than covered in our BrandyGames guide.  Between them, there's one page of merchandise catalogue with things like keychains and pencilboards and stickers, and other odds and ends.
 

Of the last two, I can only say from last summer's memory of the Digicube 'site, before it went down.  One was a book on the animation.  The other, I don't know at all - now that I think, it could have been the Special Edition of the DVD set.  I can't be sure.  Damn memory.

Anyway - The first two are still orderable on Amazon.co.jp for around 2300 Yen a piece.  I imagine that they're elsewhere, too, but I couldn't tell you more.