I always liked 'thematic books'. By this, I mean something removed from the game for themes. For eg I did not like the 'Complete...' series, where they inc bits for wizards in the rogue/scoundrel or warrior ones, BUT I did like the Book of Battle (or whatever it was called) and the Book of Horror. Both needed more and could have been if they were more like the terrain series books. My favourites were the 'Stormwarack' and 'Sandstorm' books. I know exactly where to go when our campaign is going to be based upon the seas or in a desert.
I would love for more thematic books like these. I was waiting for the mountain, forest and swamp-styled books too.
I guess, their 'story' arcs are kind of like that, but I would rather see themes used more widely and it is CLEAR when you would want this book.
And, yes, I would include the book on aberrations and the 'Dracinomicons' and the undead books in this. Those were some of my favourites too.
I am also happy with Settings as 'themes'. I always write my own settings, but love stealing from the published ones

(Though it hurts when it goes the other way, like Golarion now being a prison for a mad god - the one central hook for our big home brewed world of near on 20 yrs ;().
So, back on topic: Themed books. (Though I know everyone will not agree with this). I just want books that make it very clear when you would use/want it.