Our book lists are more geared towards every individual book released by an author. We feel adding omnibus books that just gathers the same books into one book would cause confusion. It would also make the book lists a lot messier, due to so many omnibus books out there. If there is an omnibus book by an author but it includes a novel that they haven't released individually, we will list that novel and link to the omnibus.
If you wish to use the "My Library" feature of Book Notification to track books you own, and would like to list any omnibus books you own then we have good news for you: in the future, we plan on allowing users to add their own books within "My Library". So if you own the Jack Reacher omnibus book which contains both Killing Floor and Die Tryimg and would like to log that you own that in your library, you will be able to do so with ease. We are not sure when this ability will be released at this time.