Applying a fortune II book to a diamond pickaxe has no change in drop rate of diamonds, coal, or any other items.  I have attempted multiple mines and not getting any bonus drops.

The book was found in a strong hold chest if that has any bearing on the enchant issue.  This was applied and tested in a survival map seed 1769959536, strong hold was located at 146, Y, 1426 with book in library chests.