Best monitor for bookcase ?

ATC SCM12, will do very nicely, just make sure the shelf is tight or weighted down.

ACI has custom versions of a number of their speaker where the crossover is tweaked to compensate for the baffle effect of being in a bookshelf.

I'm using a pair of Harbeth HL-P3ES-2 monitor speakers in a similar situation. Although they are not inside a bookcase, they are sitting on the top shelf of a bookcase against wall corners. They are a sealed design and, I believe, are even marketed to some degree as 'bookshelf' speakers. Great sounding little speakers with a devoted (and well justified IMHO) following.

allison fours list of $2450.00, used AR-M1

I heard a pair of Induction Dynamics bookshelf speakers crammed right back against the wall, with books packed in all around them. And they still sounded incredible! For whatever reason, these speakers don't seem to mind having walls/books in close proximity.