"Serious" clues in book three

Some mysteries, indeed my favorite ones, are the ones of which you say to yourself "I should have seen that - the author practically gave it away!" Such was my reaction upon my latest reading of "H. Potter: Prisoner of Azkaban." I realized that within the first several chapters there were enough clues delivered that, along with other facts which a Harry Potter fan would know, it could be reasoned that 1) Sirius Black was the large black dog and possibly 2) He was a friend of Harry's, not an enemy Now maybe a lot of people did figure this out off the bat? I surely didn't - but afterwards I thought that the line of reasoning could go like this when first confronted with the black dog: 1) JKR has affirmed that all names mean something. 2) Sirius is the "dog star". 3) From chapter 1 of the first book we know that sometimes wizards become animals. 4) Harry sees a black dog standing looking at him. So that seems like enough to suggest that Sirius is the "black dog" and his non-menacing behavior of merely watching Harry would suggest that he is a type of guard dog. Of course, JKR ingeniusly throws all the stuff in about the "Grim" to sow doubt about the dog's corporeal existence. But is there not a recurring theme in all the books of people not being what they seem? (E.g., Quirrell, Lockhart, Tom Riddle, (fake) Moody, Black, "future" Harry mistaken for his dad, Rita Skeeter, Scabbers/Pettigrew, Krum - and of course Snape, for whom the verdict is still not in....)