In the newest snapshot, (19w12b) you are unable to write in books and quills.
You will be able to write on the first page but once you move to the second page, when you press space it moves to the next page making it impossible to write.
I really do hope this issue is resolved before 1.14 is released.
Thank you for your report!
We're actually already tracking this issue at MC-145798, so we will resolve and link this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
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