All these books are free to read: either they're distributed with a free license, or they're in the public domain.
These Licence Art Libre licensed ebooks are free to read - the people who created them want you to read and share them.
Avec la Licence Art Libre, l'autorisation est donnée de copier, de diffuser et de transformer librement les œuvres dans le respect des droits de l'auteur.
These books are included in the Directory of Open Access Books. This means that they have been peer-reviewed and are of interest to scholars.