How I tested:

• in Creative mode created two identical books, using book and quill's one after the other.
• Named both books 'test', with text 'test' (with no spaces before or after), signed them.
• They both are labelled 'by Author Unknown'. 
• Tried to stack them in my inventory, they won't. 
• Opened and 'read' each book in case it helps to set an NBT tag.  Tried to stack again, they still won't.

According to https://minecraft.gamepedia.com/Written_Book, written books should stack into stacks of 16.  They do in Java edition.  A function ImpulseSV made use of (as a unique door key only his user can author on a multiplayer server) in his 'Door Lock with Private Key' (also used for his door into i-trade in current Hermitcraft S6) https://www.youtube.com/watch?v=5sDv37gkIlw.  The written book is filtered by a hopper and the door is opened only if the book is identical to others stacked in the hopper (only possible as the books are stackable),

Before posting, I searched but didn't find a bug report on this issue.  Found this one MCPE-35497 about books not stacking but that's regarding (blank) books, not written books.  (Books stack fine now, that old bug report must have been fixed by the way and should be closed).

Thanks to Jaren Zimmerman for pointing out this problem in comments of Impulse's tutorial (linked above) on youtube.  Many other people, in the comments, are asking if the door lock key system, will work on Bedrock Edition too.

(Find it interesting, too, there are no entries on the 'written books' wiki page (linked above), under the history section for Bedrock edition.  (No mention of when written books were added nor any changes made). 

Also, in the wiki, no mention of the read NBT tag that ImpulseSV mentioned, gets added once a book has been read for the first time.  Apparently, if a book is unread it won't stack with read books).