paperback vs travel book

paperback

noun
  • A book with flexible binding. 

adj
  • Having flexible binding. 

verb
  • To issue or publish (a book) in a paperback edition. 

travel book

noun
  • A book about travelling to a certain region, such as a memoir or guide book. 

How often have the words paperback and travel book occurred in a corpus of books? (source: Google Ngram Viewer )