People ssay they use buttercream for book cake, what kind of buttercream is it?

Buttercream is an icing consisting of fat (butter &/or Crisco), powdered sugar and liquid.
There are tons of recipes out there tht one can use. It depends on your tastes.
A forum member (Sugarshack) has a very popular on. You can find it in the recipe section of this site.

i shouldnt use crusting buttercream for ca cake? or can i?

mine is crusting and i use it under my fondant for my book cakes.