I'd also like to see how the portrait thing works they've always been around but now that a previously living character is in one I hope the idea is going to be expanded upon.
In my current slow re-read of the book I have been pondering the Half Blood Prince's book and it made me think of a few things (which may be utter waffle but hey). An old potions book belonging to the long time potions master of Hogwarts, full of his own additions is lying carelessly in the potions cupboard. A little odd no? You'd think that Snape having been there all those years would have either kept his old book full of his scribblings or at least found it again later. At the very least you would expect him to know the contents of his potions cupboard particularly when one old potions book is full of his old (and slightly dangerous) scribblings. If he did know then I am supposing that Snape knew Harry would be eligible for potions along with Ron now that Slughorn was the new teacher - Harry (trouble magnet) was therefore bound to be in contact with this carelessly strewn book. For one of the following reasons:
a) to teach him more potions - doubtful given his past teaching techniques though I suppose he could be the master of reverse psychology
But it may be to help him once he has left Hogwarts and is in pursuit of Voldemort and the death eaters to know non-curriculum magic. I don't know how much use potions is but as we are constantly reminded there are poisons without cures etc, and Harry's new found (previously taught and forgotten) knowledge does save Ron's life. Harry also seems to have found a new enthusiasm for potions through the book.
b) to help him get a good grade in Potions - could Auror training help Harry more after school? Considering Harry is not very well practised in non-verbal spells I am assuming there is a lot more for him to learn. Perhaps the next book will be set in the Auror world of training as it would make sense for him to learn a
lot more magic before the next show down.
c) to get in Slughorns good books - Dumbledore has told Snape of his plans for Harry to retrieve information willingly from Slughorn. Snape is as ever doubtful of Harry's talents and so throws him a bone.
or d) the book is not meant for Harry but Malfoy. Snape did not think that Harry or Ron would be taking potions due to their grade and as part of his mission to help Malfoy when the kid doesn't even want his help he plants the book in the class room. We already know it contained one nasty curse and maybe there was more besides.
Or Snape does not know the book is there which for the reasons at the beginning of my post would be rather odd. Surely he would have noticed? Unless the book was put there after Snape left his lair. Slughorn planted the book perhaps, though he does a good job of pretending he doesn't know that. But then he is a master of concealment. If Snape did know I suppose it would support the theory of Snape being on the good side of the narrow line (unless it was for Malfoy in which case it doesn't really settle it one way or the other), if he didn't know then perhaps there is more to Slughorn (or someone else) than we have been allowed to know so far.
Or maybe, just maybe, it means nothing at all