Is the book, the library book for healing hands?
If so, the book is actually on top of one of the four bookcases in the first section of the library. Now will come a bit of a puzzle. When you see the book, you need to climb on a table and then onto the bookcase, the book will fly away. Now, move the table using Wingardium Leviosa to the bookcase the book has flown too, and climb up onto the bookcase again, the book will yet again fly off. Repeat this process until the book finally flies onto a table in the middle of the room. Now you can jump down and retrieve the book from that table. After retrieving the book, you will need to travel to Moaning Myrtles bathroom to read the book in peace.

Good luck...