Written Books (minecraft:written_book item) and Book and Quills (minecraft:writable_book item) without their respective book content components (minecraft:written_book_content and minecraft:writable_book_content components respectively) can still be placed on Lecterns.

This causes different behaviour depending on whether you're in 1.21.1 or the latest 1.21.2 pre-release (4 as of writing), but neither seems fully intentional, and the 1.21.2 behaviour is somewhat problematic.

1.21.1 Behaviour

In 1.21.1, placing these books on a Lectern will allow you to open the book GUI, showing a blank book. This is inconsistent with the item form of the Written Book, which will not open a GUI when used if it is missing its book content component. (Though notably consistent with the Book and Quill item)

1.21.2 Pre-Release 4 Behaviour

In 1.21.2 Pre-Release 4, placing these books on a Lectern will allow you to open the book GUI, showing a blank book... only for the GUI to close again nearly immediately. This is not only inconsistent with the item forms of both books (both of which will not open a GUI when used if missing their book content component), but more pressingly results in being unable to use the 'Take Book' button in the GUI to retrieve the item, requiring instead that the Lectern being broken.

How to Reproduce

1. Obtain a Written Book without the minecraft:written_book_content component, using the command /give @s minecraft:written_book[!minecraft:written_book_content] (alternatively, use the command /give @s minecraft:writable_book[!minecraft:writable_book_content] for a Book and Quill)
2. Observe that right-clicking to attempt to open the Book item does not open the Book GUI without the component (unless in 1.21.1 with a Book and Quill)
3. Place down a Lectern, and use the book on the Lectern
4. Observe that, despite not having the book contents component, the book can be placed on the Lectern
5. Right-click to use the Lectern
6. Observe the GUI behaviour. If in 1.21.2 Pre-Release 4, note the GUI closes nearly immediately, preventing the Take Book button from being used. Regardless, note the behaviour being different to using the book item earlier. (Again, unless using specifically a Book and Quill in 1.21.1)

Proposed Solution

It is my opinion that items without either the minecraft:written_book_content or minecraft:writable_book_content component should not be able to be placed on a Lectern at all, which would solve this issue.

In fact, this restriction could outright replace the minecraft:lectern_books item tag, allowing for any item with these components to be placed on a Lectern, which would allow for custom book items to be made using commands without a data pack.