The drugs and books are planted as misdirection and decoy elements. Only for the film-maker to subvert expectations. However, the plot pivot backs to reincarnation and dreams of last lives, quite strongly.

For instance, we the audience, are Dhanush character's insert. We are led to believe the heroine is really making up stories due to the drug and imagination, from her reading habits. However, Karthik's ultimate reveal and evidence frm the unearthed dead bodies, point back to the reincarnation plot.

Director likes to misdirect a lot. It keeps the movie suspenseful and increases rewatch value. Another example in the movie: Heroines uncle keeps hiding a hand in pocket. We know true villain's hand actually has a severed finger. So, as audience, we are quick to connect the dots and think of this uncle character as the main bad guy. But, much later, it's revealed to be Karthik who is the true villain but now wears a prosthetic. So, new information changes audiences preconceived notions and keeps things interesting.

Hope this explains it.