About Shelob: Here she is described as coming from ruin to her lair which presumably occured at the time of the down fall of Ungoliant and Morgoth. It is interesting to note that both her coming and ending in the tales by Tolkien are mysterious and left open to the interpretations of the reader...

"There age long , she had dwelt, an evil thing in spider form, even such as once of old had lived in the land of the elves in the west that is now under the sea, such as Beren fought in the mountains of terror in Doriath, and so came to Luthien upon the green sward amid the hemlocks in the moonlight long ago. How Shelob came there, flying from ruin, no tale tells, for out of the dark years few tales have come. But still she was there, who was there before Sauron and before the first stone of Barad-dur; and she served none but herself, drinking the blood of elves and men, bloated and grown fat with endless brooding on her feasts, weaving webs of shadow; for all living things were her food, and her vomit darkness. Far and wide her lesser broods, bastards of the miserable mates, her own offspring, that she slew, spread from glen to glen, from the Ephel Duath to the eastern hills, to Dol Guldur and the fastnesses of Mirkwood. But none could rival her. Shelob the great, last child of Ungoliant to trouble the unhappy world."

Also;

"And as for Sauron; he knew where she lurked. It pleased him that she should dwell there hungry but unabated in malice, a more sure watch upon that ancient path into his land than any other that his skill could have devised. And orcs, they were useful slaves, but he had them in plenty. If now and again Shelob caught them to stay her appetite, she was welcome: he could spare them. And sometimes as a man may cast a dainty to his cat (his cat he calls her, but she owns him not) Sauron would send her prisoners that he had no better uses for: he would have them driven to her hole, and report brought back to him of the play she made. So they both lived, delighting in their own devices and feared no assult, nor wrath, nor any end of their wickedness. never yet had any fly escaped from Shelob's webs, and the greater now was her rage and hunger."

Regarding whether Shelob dies or not is not actually mentioned. All it says is this after Sam defeats her:

" Shelob was gone; and whether she lay long in her lair, nursing her malice and her misery, and in slow years of darkness healed herself from within, rebuilding her clustered eyes, until with hunger like death she spun once more her dreadful snares in the glens of the Mountains of Shadow, this tale does not tell."

So Tolkien has left the ending of Shelob open. It is up to the reader to imagine what may have or may not have happened. I dont think Tolkien wished for intellectual debates on the matter. Rather, as a fantasy novel, it is for the world of imagination in the mind and thoughts of the reader to decide. Thats what fantasy is about and perhaps thats why its written in this way. Just a thought to ponder...

Source: The Two Towers, ch1X, Shelob's Lair by JRR Tolkien