Libre but not free books  
  Definition  
  A book is libre if its authors have renounced to their patrimonial rights (no remuneration). But they keep all their moral rights and should be cited.
  Libre books are available on free numerical form (pdf...) and often on paid printed forms (for printing and distribution costs).
  
  Examples of libre but not gratuitous books  
  
    - Framabook  (https://framabook.org)
    
 
    - SageMath  (http://www.sagemath.org) : a free open-source mathematics software system licensed under the GPL.
    
 
  
  
  A new approach : classical books + free-libre supplements ?