book.rb

class Book < ApplicationRecord
  validates :title, presence: true
  validates :author, presence: true
  validates :genre, presence: true
  validates :classification, presence: true, :inclusion { :in => @allowed_classifications }


  @allowed_classifications = [
   'General Works - encyclopedias',
   'Philosophy, Psychology, Religion',
   'History - Auxiliary Sciences',
   'History (except American)',
   'General U.S. History',
   'Local U.S. History',
   'Geography, Anthropology, Recreation',
   'Social Sciences U',
   'Political Science V',
   'Law Z - Bibliography and Library Science',
   'Education',
   'Music',
   'Fine Arts',
   'Language and Literature',
   'Science',
   'Medicine',
   'Agriculture',
   'Technology',
   'Military',
   'Naval Science',
   'Bibliography and Library Science',
   'type'
  ]

end

I recommend using a class method called classifications instead of an instance variable here which returns an array. Also, you're missing the book_type and year validations.