i'm sorry I do believe he will be seen in the next book somehow though...

No worries. And I feel the same way. There's something about his death that doesn't seem right. He was never even given a chance to do anything worthwhile before JKRowling killed him off. It sort of makes book 3 completely worthless - what was the point in freeing him, giving Harry a hope of a new home? He's gone now!

Which brings me to the point I have to agree with you, Pandora. We did not actually see Sirius die, or see a body. Unlike Cedric, where we were certain he had died. Nor do we fully understand what this 'veiled curtain' was, what was behind it, and why it was there.

And what's a Sirius Tread without his image on every page

plus Bellatrix only used a stunning spell not a killing spell

Order of the Phoenix, Chapter 35:-

Only one pair was still battling, apparently unaware of the new arrival (Dumbledore). Harry saw Sirius duck Bellatrix's jet of red light: he was laughing at her.
'Come on, you can do better than that!' he yelled, his voice echoing around the cavernous room.
The second jet of light hit him squarely on the chest.
The laughter had not quite died from his face, but his eyes widened in shock.
Harry released Neville, though he was unaware of doing so. He was jumping down the steps again, pulling out his wand, as Dumbledore, too, turned towards the dais.
It seemed to take Sirius an age to fall: his body curved in a graceful arc as he sank backwards through the ragged veil hanging from the arch.
Harry saw the look of mingled fear and surprise on his godfather's wasted, once-handsome face as he fell through the ancient doorway and disappeared behind the veil, which fluttered for a moment as though in a high wind, then fell back into place.
Harry heard Bellatrix Lestrange's triumphant scream, but knew it meant nothing - Sirius had only just fallen through the archway, he would reappear from the other side any second...
But Sirius did not reappear.
'SIRIUS!' Harry yelled. 'SIRIUS!'
He had reached the floor, his breath coming in searing gasps. Sirius must be just behind the curtain, he, Harry, would pull him back out...
But as he reached the ground and sprinted towards the dais, Lupin grabbed Harry around the chest, holding him back.
'There's nothing you can do, Harry - '
'Get him, save him, he's only just gone through!'
'- it's too late, Harry.'
'We can still reach him - ' Harry struggled hard and viciously, but Lupin would not let go...
'There's nothing you can do, Harry ... nothing .. he's gone.'

I also heard an interesting theory on mugglecast that the final Harry vs Voldie fight is going to happen in the department of mysteries as love is the only thing that can defeat Voldie and Dumbledore said that there's a room devoted to love there. So if they are in the ministry I suspect the veil will come into it too.

'Come on, you can do better than that!' he yelled, his voice echoing around the cavernous room.
The second jet of light hit him squarely on the chest.
The laughter had not quite died from his face, but his eyes widened in shock.
Harry released Neville, though he was unaware of doing so. He was jumping down the steps again, pulling out his wand, as Dumbledore, too, turned towards the dais.
It seemed to take Sirius an age to fall: his body curved in a graceful arc as he sank backwards through the ragged veil hanging from the arch.
Harry saw the look of mingled fear and surprise on his godfather's wasted, once-handsome face as he fell through the ancient doorway and disappeared behind the veil, which fluttered for a moment as though in a high wind, then fell back into place.
Harry heard Bellatrix Lestrange's triumphant scream, but knew it meant nothing - Sirius had only just fallen through the archway, he would reappear from the other side any second...
But Sirius did not reappear.
'SIRIUS!' Harry yelled. 'SIRIUS!'
He had reached the floor, his breath coming in searing gasps. Sirius must be just behind the curtain, he, Harry, would pull him back out...
But as he reached the ground and sprinted towards the dais, Lupin grabbed Harry around the chest, holding him back.
'There's nothing you can do, Harry - '
'Get him, save him, he's only just gone through!'
'- it's too late, Harry.'
'We can still reach him - ' Harry struggled hard and viciously, but Lupin would not let go...
'There's nothing you can do, Harry ... nothing .. he's gone.'