A Grave Mistake

The year is 1981, August 6. Sirius Black, James Potter, Lily Potter (Nee Evans) are all sitting in the living room of the Potter's residence in Godrics Hollow but soon enter the kitchen however, when they were about to dig in, a sudden flash of bright Red light engulfed them, a book appears on the kitchen table In front of them all.
The book is called;
Harry Potter and the Prisoner of Azkaban.
What is this book? and why did it come with a note asking for only the three of them to read it and request that they re-think their decisions about who they trust? And why does it say to not tell anyone especially not Peter Pettigrew or Remus lupin about the book?