Originally Posted by enkov
I think Index is common for all books, not for every single book. ...
The Kindle makes separate index files for each book plus two additional files which likely are top level master indexes. They are stored in the "/system/Search Indexes" folder.
I backup my complete Kindle by simply copying all visible files (I use Windows with "show hidden files and folders" turned on) to a storage folder on an external hard drive. As yet, I haven't needed to do a restore so I can't say how complete a recovery would be. I would suspect that a complete overwrite of the Kindle documents folder and all of the index files should prevent the need to reindex.