Shouldn't some of the methods of kiwix::Library be const?

[veloman-yunkan]

The following public methods of kiwix::Library are conceptually const but are not declared such:

  std::shared_ptr<Reader> getReaderById(const std::string& id);
  bool writeToFile(const std::string& path);
  bool writeBookmarksToFile(const std::string& path);
  unsigned int getBookCount(const bool localBooks, const bool remoteBooks);
  std::vector<std::string> getBooksLanguages();
  std::vector<std::string> getBooksCreators();
  std::vector<std::string> getBooksPublishers();
  const std::vector<kiwix::Bookmark> getBookmarks(bool onlyValidBookmarks = true);
  BookIdCollection getBooksIds();
  DEPRECATED BookIdCollection filter(const std::string& search);
  BookIdCollection filter(const Filter& filter);
  void sort(BookIdCollection& bookIds, supportedListSortBy sortBy, bool ascending);
  DEPRECATED BookIdCollection listBooksIds(...);

Besides, it makes sense to have const versions of the following methods:

  Book& getBookById(const std::string& id);
  Book& getBookByPath(const std::string& path);

Is there anything that is preventing us from doing this?

[mgautierfr]

Is there anything that is preventing us from doing this?

No.
Potentially API/ABI compatibility, but we can bump the version.