I see here MS have published the specification for the BIFF (.xls) file format. Just by a quick glance it looks they have also released the first ever info on the mysterious .xlb toobar files. The file format stuff is updated from 97 I think, which was the last time it got published.
I wouldn’t rush to print it though, its a thousand pages!
Blimey, they have also published the format of VBA projects, another first.
Are you planning any apps that use the file formats directly?
(I have one for removing VBA projects, but its a bit aggressive – I can now find out where I should be deleting from, hopefully).