You have a book with Pages A > Z
1) You will work with the Saved Book Collection, (NOT a standard collection you might have used initially..)
2) Duplicate the Saved Book Collection A>Z in the Collections panel. [Right-Click] and [Duplicate the book] - "...Copy"
3) Duplicate the Saved Book Collection A>Z a second time. "...Copy 2".
Now you have three versions the same- The Original A>Z kept for 'backup' and two copies.
4) Edit Book "...Copy" and delete pages M > Z
5) Edit Book "...Copy 2" and delete pages A > L
6) Rename the copies as suitable- eg. Part 1, Part 2.
Result you have two books (A>L) and (M>Z) to edit individually. (as well as you original A>Z book for security).
