First sentence means: You started reading a book sometime ago and are still reading it. Second sentence means: you started and finished the book sometime ago. Good question, hope this helps.

"have been reading" means until now "had been reading" means until some moment in the past

have been reading (present perfect continuous): action started in the past and it's true in the present. had been reading (past perfect continuous) : action started in the past and also finished in the past.

this action beginning in the past didn't end in the present