I don't know, but aren't aberrations quite a loose category to get a book like this?
My first idea was that it could be a book about fiends, because just like dragons and undead they are a popular group, broad enough to easily generate new creatures but also a cohesive group.
It is easy to conceive the same sort of material that is in Draconomicon and presumably Libris Mortis: guidelines about playing a fiend PC, prestige classes for fiends and for normal characters who fight against fiends or who want to become close to them, magic items related to fiends, lots of flavor text about the phisiology and ecology of fiends (although obviously totally unearthly).
Could you do the same for aberrations? Sure, but it'd be more difficult IMO not to turn the book into simply a collection of monsters entries...