Created Books do not appear in the Book list

There is also another problem which probably relate to the preceding one : All book pages created using the "Add child page" link and set by default with the current book page as parent lost the "Parent:" setting after they've been saved. When we go back to edit them they return to "top-level" and it is not possible to select back the real parent since it is not available in the choices, in fact, only "top-level" is available.

Though, the links at the bottom (parent page link and the "up" link) of the page are good and they point to the right parent.