As for the .t2b Files. Those are - as HarryT pointed out - the thumbnails files created and used by the CyBook.
If you have an eBook with no cover or the cover integrated in the eBook is not to your liking you could replace the .t2b file that belongs to the book in question with one you like better.
Because I had that need I tried to find information about the .t2b file format. Yet, I could not find anything and came to the conclusion that this must be some proprietary Bookeen format. Stubborn that I am this didn't hold me back however.
Hence I created this :
Which you can download from here:
To run this little tool you need the .net 2.0 Framework installed on your computer. If any Linux or Mac User is interested, I might take the time to check if this also works with the Mono Framework and / or compile it against the mono framework if I find the time.
(Almost goes without saying: this little piece of software is free, is provided as is, and I can guarantee for nothing but that it is free of mal- or spyware)