Signed book are actually writable! but however if you attempt to edit the text, it won't work at all.
Android Device: MOBIISTAR_LAI_ZORO
What I expected to happen was...:
Nothing happens. since it is uneditable.
What actually happened was...:
It brings up the keyboard, allowing you to edit the text.
Steps to Reproduce:
1. Create a new world (or load a existing one).
2. Get a 'Book and Quill'.
3. Enter any text in a 'Book and Quill' and them sign it. (any title you wanted to be)
4. Open that Book that you've signed it recently and tap on the text.
PS. i tried to search "book", "writable book" and "signed book" on this website but it came up just a very old bugs and sometimes no results. So sorry if this is a duplicate bug.