I am jumping around in your Help here, so don't expect any orderly notes
Observations (I use Firefox 12 if that matters

)
Main Page: I clicked on the blue Metadata in the middle of the page (I saw
a change) I had to click cycle a couple of times to figure out what changed. I don't think adding an inline expansion in the same style is obvious enough
(And you thought you were going to get a Calibre critique

)
About the Calibre Library:
(wording nit pick)
Code:
Caution: Do NOT change the calibre library structure.
calibre is a database, Files and folders are part of the Database.
Any changes to the library file or folder namestructure will break the database, and calibre won't be able to find your books.
(nit as this applies to a GUI user)
Code:
When calibre converts a file, it creates a new folder in the directory that you specified when you set up calibre with the welcome wizard.
When you add a book, Calibre creates a folder for the book inside the Author folder. If the Author folder does not exist, it will be created.
Any conversions on that book, will be placed or replaced within that folder.
FYI
Code:
C:\Users\xxxx\MyDocuments\Calibre Library\author name\book name (calibre DB index number)
. The Index number is what ties this folder to a particular (version of the) book listing in the Calibre.
(nother nit) Metadata
A backup of the metadata
on 'About Calibre: '
Click here
for to hear the proper pronunciation of calibre.
(do you have a speaker/sound icon that could be used instead of 'here'.
I have run out of comment steam and time..