Cannot programatically update books

I think there maybe a little confusion in testBookApi()

the call to createBookNode() with $parent, in effect, set to NULL means that

     $this->drupalPostForm('node/add/book', $edit, t('Save'));

will be called...when createBookNode exits $book->save() is called on an already saved node ?

This makes no sense to me

Ah good point

What is critical about this?

Only fatal errors that can be triggered in the UI are critical, and actually, not even that is mentioned anymore on https://www.drupal.org/node/45111, and it also doesn't fall into any of the categories there?

Just a note on this, may help to find a solution:

$node = Node::load(nid);
// The form code does something like this.
$node->book['original_bid'] = $node->book['bid'];
$node->save();

This works and any change on $node is save.

book never promised to be pretty. I did try to play with the logic a bit but there isn't a lot you can do (I tried to replace $node->book['original_bid'] = $node->book['bid']; with simply $new = TRUE and adjust $new accordingly) but this fails. This patch works so let's do it.

I wondered if could we remove the corresponding lines from book_node_prepare_form(). Yep this is ugly but this is not the worst thing in the Book module.

+++ b/core/modules/book/src/Tests/BookTest.php
@@ -65,6 +65,15 @@ protected function setUp() {
   /**
+   * Test that we can save a book programatically.
+   */
+  public function testBookApi() {
+    $this->drupalLogin($this->book_author);
+    $book = $this->createBookNode('new');
+    $book->save();
+  }

Let's not do a whole Drupal install just to test this. Perhaps we can add this to another test method. How about testBook() since that Tests book functionality through node interfaces.

For #14

OK that's enough of attempting this. #16 is RTBC. It's not the role of this issue to fix book.module.

This issue addresses a major bug and is allowed per https://www.drupal.org/core/beta-changes. Committed #16 a27d222 and pushed to 8.0.x. Thanks!