How to design a graph database in this scenario?
Should I create 1 MandatoryData for each book?


Yes.

If you cannot get rid of MandatoryData object, you should have one such object per book.

Otherwise I would simplify the model and move any mandatory attributes to the Book object, which will reduce ne04j database size. Modeling relationships which are always 1:1 is possible, but not optimal. You can improve query performance by keeping the data in one object.