Unfortunately, I believe that book will play a heavy role in Astarion's quest, or at the minimum... What happens to his alignment later. If he can master his vampirism, he will not need the tadpole anymore. Without that, I'm not sure there's a good ending for Astarion.
Well, you could be right of course, but I would be very surprised if the book were essential to achieving a positive outcome for Astarion, at least unless the writing in the full release is changed to signpost this much more clearly. Right now, my guess is that giving him the book can help him get a happy ending (as it could help Gale), but that there will also be alternative ways of getting there if a different choice is made with respect to the Necromancy of Thay.
