There is only one file type that uses the .KOOB extension:
Koob E-Book File
E-book file created by Koober, an application used to create and read e-books in the Koob format; contains book chapters, page navigation data, book styles, publication data, and other e-book information; can be read with KoobReader, a free e-book reading application included with Koober.
NOTE: The term "koob" is backward for "book."
Updated: September 10, 2011