Why leave the book?

[potioncat]

No: HPFGUIDX 136974

 
> CathyD now:
 Snape had pretty much given up on getting Harry expelled by GOF (in 
fact, did he threaten expulsion in GOF?  He threatened veritaserum
once 
and told him he would *pay* but he never said expel).  Snape only
gave 
Harry detention for nearly murdering Malfoy.  That was grounds for 
expulsion if anything was. Was Snape trying to help Harry?  I don't 
know, but Harry finally learned Potions from Snape, didn't he?  (I 
think you can tell this is a big plot-hole to me...I just can't get 
around it.  

Potioncat:
I may be repeating myself...but at my age, it's to be expected and to
a 
certain extent, tolerated.

Yeah, even when there was a headmistress who would have gladly 
expelled Harry, Snape made no attempt at it. I think it was always 
intended as bluster.

Snape lost his book years ago; or Snape's book was stolen years ago.
If 
the book has been in the Potions cabinet in the class room all this 
time, it's because someone quietly put it back without Snape knowing. 
He's the type of teacher who wouldn't offer an old book to a student 
to use. 
It's also possible that Slughorn has has this book all these years in
a 
box somewhere and didn't know what he had.

Was it slipped to Harry on purpose? Well, possibly Slughorn did, if
he 
had some reason...perhaps to make Harry look like a better student
than 
he had been. Was it dumb luck? Harry has a lot of that! I don't think 
Snape had anything to do with it, and I wouldn't think DD would want 
that sort of Dark Magic floating around either.

Snape is surprised that Harry is doing well in Potions. But he is 
furious that Harry knows the sectumsempra spell. He demands to know 
where Harry learned it. Later he is angry that his spells are being 
used against him. Clearly, Snape never intended any student to get 
ahold of the book.

He also says that James used his spells. If the spells are non
verbal, 
how did James learn them? I think someone found/took the book and
used 
the spells, perhaps sharing them with his best friend. The biggest
hole 
in my idea, is that James knew the Levicorpus spell in 5th year. That 
seems too early for the book to go missing.

I think it is very interesting that Hermione who always trusts Snape 
does not trust the HBP; and the boys who always loathe Snape admire 
the HBP. Someone else brought that up in another thread, but I 
thought I'd repeat it....comes with age, you know.